Vertical cavity surface emitting laser device, vertical cavity surface
emitting laser array, optical scanning apparatus, image forming apparatus, optical transmission module and optical transmission system

ABSTRACT

A disclosed vertical cavity surface emitting laser device emits light orthogonally in relation to a substrate and includes a resonator structure including an active layer; and semiconductor multilayer reflectors disposed in such a manner as to sandwich the resonator structure between them and including a confinement structure which confines an injected current and transverse modes of oscillation light at the same time. The confinement structure has an oxidized region which surrounds a current passage region. The oxidized region is formed by oxidizing a part of a selective oxidation layer which includes aluminum and includes at least an oxide. The selective oxidation layer is at least 25 nm in thickness. The semiconductor multilayer reflectors include an optical confinement reducing section which reduces optical confinement in a transverse direction. The optical confinement reducing section is disposed on the substrate side in relation to the resonator structure.

TECHNICAL FIELD

The present invention is directed to a vertical cavity surface emittinglaser device, a vertical cavity surface emitting laser array, an opticalscanning apparatus, an image forming apparatus, an optical transmissionmodule and an optical transmission system. In particular, the presentinvention is directed to a vertical cavity surface emitting laser devicewhich emits light orthogonally in relation to a substrate; a verticalcavity surface emitting laser array in which such vertical cavitysurface emitting laser devices are integrated; an optical scanningapparatus including such a vertical cavity surface emitting laser deviceor such a vertical cavity surface emitting laser array; an image formingapparatus including such an optical scanning apparatus; and an opticaltransmission module and an optical transmission system including such avertical surface emitting laser array.

BACKGROUND ART

Because of their structures, vertical cavity surface emitting laserdevices are characterized by ease of lowering the threshold current andthe power consumption. In recent years, oxide-confined vertical cavitysurface emitting laser devices have been intensively studied, whichdevices allow lowering the threshold current and provide a higher-speedresponse compared to ion-implanted vertical cavity surface emittinglaser devices that were previously studied (see Non-patent Document 1,for example).

Oxide-confined vertical cavity surface emitting laser devices have anadvantage of having favorable transverse mode confinement provided by anoxide, which results in a stable oscillation mode; however, since theoptical confinement by the oxide is too strong, it is difficult toobtain a single fundamental transverse-mode oscillation. Note that anoxide-confined vertical cavity surface emitting laser device is referredto simply as “vertical cavity surface emitting laser device” below.

A widely adopted conventional technique for achieving a singlefundamental transverse-mode operation is to provide a small area of anunoxidized region, which is a current injection region (current passageregion), so that higher-order transverse modes are confined and do notoscillate. In other words, the technique is to cut off higher-ordertransverse modes.

Another proposed method for achieving a single fundamental transversemode operation is to reduce the strength of the transverse modeconfinement provided by an oxide. If the strength of the transverse modeconfinement is reduced, higher-order mode oscillation is suppressed. Inthis case, there is no need to make the area of the unoxidized regionsmall, and therefore, both thermal and electrical characteristics areimproved. This results in an increase in the saturation power and alsoan increase in the modulation rate. In order to reduce the strength ofthe optical confinement by an oxide, conventionally, the oxide isprovided at a position away from the active layer, or the oxide is madeto be thin.

Vertical cavity surface emitting laser devices may be readily arrangedin two-dimensions at high density since each laser device emits laserlight orthogonally in relation to its substrate. Accordingly, theirapplications to high-speed and high-definition electrophotographicsystems and the like have begun to be explored. For example, Non-patentDocument 2 discloses a printer using a 780-nm band VCSEL array (verticalcavity surface emitting laser array). Patent Document 1 discloses amulti-spot image forming apparatus having a multi-spot light source. Ingeneral, higher-speed optical writing can be achieved by using avertical cavity surface emitting laser device capable of performinghigh-power operations in a single fundamental transverse mode.

Such a vertical cavity surface emitting laser device includes a currentconfinement structure in order to increase the efficiency of currentinflux. A commonly used current confinement structure is formed throughselective oxidation of an AlAs (aluminum arsenide) layer (the currentconfinement structure is also referred to as “oxide current confinementstructure” below) (see Patent Document 2, for example). An oxide currentconfinement structure is obtained by forming, in a precursor structure,a mesa of a predetermined size, in which a p-AlAs layer to beselectively oxidized is exposed along the lateral sides and placing theprecursor structure in a high-temperature water vapor atmosphere so thatAl is selectively oxidized from the lateral sides in such a manner thata central portion of the mesa remains unoxidized. The unoxidized portionfunctions as a passage region (current injection region) of the currentfor driving the vertical cavity surface emitting laser. In this way,current confinement is readily obtained.

Regarding a vertical cavity surface emitting laser, if heat generated inthe active layer is rapidly released, a rise in the junction temperature(temperature of the active layer) can be suppressed and a decrease ingain can be prevented. This leads not only to a high output but also tofavorable temperature characteristics, and hence longer operating life.

Semiconductor multilayer reflectors are in general made of AlGaAsmaterials. The thermal conductivity of an AlGaAs material largely variesdepending on the Al component, and AlAs has the highest thermalconductivity (see FIG. 65).

Given this factor, it has been proposed that each AlAs low refractiveindex layer, which is included in a semiconductor multilayer reflectordisposed on the heat release path side and is adjacent to the resonatorstructure, is designed to have an optical thickness larger than usual(see Patent Documents 3 to 5, for example).

[Patent Document 1] Japanese Laid-open Patent Application PublicationNo. H11-48520

[Patent Document 2] US Patent Publication No. 5493577 [Patent Document3] Japanese Laid-open Patent Application Publication No. 2005-354061[Patent Document 4] Japanese Laid-open Patent Application PublicationNo. 2007-299897 [Patent Document 5] US Patent Publication No. 6720585

[Non-patent Document 1] K. D. Choquette, R. P. Schneider, Jr., K. L.Lear & K. M. Geib, “Low threshold voltage vertical-cavity lasersfabricated by selective oxidation”, Electronics Letters, No. 24, Vol.30, 1994, pp. 2043-2044[Non-patent Document 2] H. Nakayama, T. Nakamura, M. Funada, Y. Ohashi &M. Kato, “780 nm VCSELs for Home Networks and Printers”, ElectronicComponents and Technology Conference Proceedings, 54^(th), Vol. 2, June,2004, pp. 1371-1375

In electrophotography or the like, a significant influence is exerted onimage quality by the rising behavior of the optical output responsewaveform of the light source, obtained when a drive current is appliedto the light source. The optical output response waveform represents thetime change in the optical output, and is hereinafter also referred toas “optical waveform”. For example, image quality may be degraded by afractional change in light intensity not only during the rise time ofthe optical waveform but also after the optical output has reachedconstant light intensity at the beginning of the rise.

This is because parts of an image formed during the rise and fall timesof the optical waveform are the contour of the image. If the lightintensity changes especially during the rise time of the opticalwaveform and during a certain time period after the optical waveform canbe regarded to have substantially risen, the contour of the imagebecomes blurred, resulting in poor image quality with lack of visualsharpness.

For example, in the case where 300 μs is required to scan one line on anA4 sheet having a width (lengthwise direction) of about 300 mm, the scandistance in 1 μs is about 1 mm. It is said that the human eye has thehighest visual sensitivity for change in image density when the width is1 to 2 mm. Therefore, if the image density changes over about 1 mm inwidth, the density change is sufficient to be detected by the human eye,giving an impression of a blurred contour.

Another problem that the present invention addresses relates to theoptical thickness of the low refractive index layers in thesemiconductor multilayer reflector. If the optical thickness of each lowrefractive index layer is changed from λ/4 (λ is the oscillationwavelength) to 3λ/4, the absorption of light (hereinafter, also referredto simply as “absorption” for convenience) is increased by three-fold).Within the semiconductor multilayer reflector, the closer to theresonator structure, the stronger the electric field intensity, andtherefore, a significant influence of the absorption is exerted. As aresult, the methods disclosed in Patent Documents 3 to 5 leave theproblem of causing a decrease in the slope efficiency and an increase inthe threshold current.

DISCLOSURE OF INVENTION

In view of a new finding of the inventors described below, the presentinvention includes the following aspects.

The first aspect of the present invention is a vertical cavity surfaceemitting laser device which emits light orthogonally in relation to asubstrate and includes a resonator structure including an active layer;and semiconductor multilayer reflectors disposed in such a manner as tosandwich the resonator structure between them and including aconfinement structure which confines an injected current and transversemodes of oscillation light at the same time. The confinement structurehas an oxidized region which surrounds a current passage region. Theoxidized region is formed by oxidizing a part of a selective oxidationlayer which includes aluminum and includes at least an oxide. Theselective oxidation layer is at least 25 nm in thickness. Thesemiconductor multilayer reflectors include an optical confinementreducing section which reduces optical confinement in a transversedirection. The optical confinement reducing section is disposed on thesubstrate side in relation to the resonator structure.

The second aspect of the present invention is a vertical cavity surfaceemitting laser device that emits light orthogonally in relation to asubstrate and includes a resonator structure including an active layerand semiconductor multilayer reflectors disposed in such a manner as tosandwich the resonator structure between them and including multiplepairs of a first layer and a second layer. The first layer and thesecond layer have different refractive indexes. The second layer hashigher thermal conductivity than the first layer. The semiconductormultilayer reflectors include a first partial reflector and a secondpartial reflector. The first partial reflector includes at least one ofthe pairs, in which the second layer is greater in optical thicknessthan the first layer. The second partial reflector is disposed betweenthe first partial reflector and the resonator structure, and includes atleast one of the pairs, in which each of the first layer and the secondlayer is less in the optical thickness than the second layer of thefirst partial reflector.

The third aspect of the present invention is a vertical cavity surfaceemitting laser array on which multiple vertical cavity surface emittinglaser devices of the present invention are integrated.

The fourth aspect of the present invention is an optical scanningapparatus for scanning a scanning surface with light. The opticalscanning apparatus includes a light source including the vertical cavitysurface emitting laser device of the present invention; a deflectorconfigured to deflect light emitted from the light source; and ascanning optical system configured to focus the deflected light on thescanning surface.

The fifth aspect of the present invention is an optical scanningapparatus for scanning a scanning surface with light. The opticalscanning apparatus includes a light source including the vertical cavitysurface emitting laser array of the present invention; a deflectorconfigured to deflect light emitted from the light source; and ascanning optical system configured to focus the deflected light on thescanning surface.

The sixth aspect of the present invention is an image forming apparatusincluding at least one image carrier; and one or more of the opticalscanning apparatuses of the present invention configured to irradiate,on the at least one image carrier, light which includes imageinformation.

The seventh aspect of the present invention is an optical transmissionmodule for generating an optical signal according to an input electricalsignal. The optical transmission module includes the vertical cavitysurface emitting laser array; and a drive device configured to drive thevertical cavity surface emitting laser array according to the inputelectrical signal.

The eighth aspect of the present invention is an optical transmissionsystem including the optical transmission module; an opticaltransmission medium configured to transmit the optical signal generatedby the optical transmission module; and a converter configured toconvert the transmitted optical signal into an electrical signal.

BRIEF DESCRIPTION OF DRAWINGS

These and other objects, features and advantages of the invention willbe apparent to those skilled in the art from the following detaileddescription of the invention, when read in conjunction with theaccompanying drawings in which:

FIG. 1 shows a schematic structure of a laser printer according to oneembodiment of the present invention;

FIG. 2 is a schematic diagram showing an optical scanning apparatus ofFIG. 1;

FIG. 3 shows a vertical cavity surface emitting laser device included ina light source of FIG. 2;

FIGS. 4A and 4B are illustrative diagrams of a substrate of FIG. 3;

FIG. 5 is an enlarged view showing a part of a lower semiconductor DBRof FIG. 3;

FIG. 6 is an enlarged view showing the vicinity of an active layer ofFIG. 3;

FIG. 7 shows an optical waveform obtained when a conventional verticalcavity surface emitting laser device is driven by a square wave currentpulse having a pulse period of 1 ms and a duty of 50%;

FIG. 8 shows an optical waveform obtained when the conventional verticalcavity surface emitting laser device is driven by a square wave currentpulse having a pulse period of 100 ns and a duty of 50%;

FIG. 9 is an illustrative diagram of a built-in effective refractiveindex difference Δneff (Part 1);

FIGS. 10A and 10B are illustrative diagrams of the built-in effectiverefractive index difference Δneff (Part 2);

FIGS. 11A and 11B are illustrative diagrams of the built-in effectiverefractive index difference Δneff obtained when an internal temperatureincreases;

FIG. 12 is an illustrative diagram of a shift of an I-L curve due to anincrease in the internal temperature of a vertical cavity surfaceemitting laser device having insufficient optical confinement in thetransverse direction at room temperature;

FIG. 13 shows an optical waveform obtained in the case of FIG. 12;

FIG. 14 shows refractive indexes used for calculation;

FIG. 15 shows the relationship among an optical confinement coefficient,thickness of a selective oxidation layer and an oxide confinementdiameter (Part 1);

FIG. 16 shows the relationship among the optical confinementcoefficient, the thickness of the selective oxidation layer and theoxide confinement diameter (Part 2);

FIG. 17 shows the relationship among the optical confinementcoefficient, the thickness of the selective oxidation layer and theoxide confinement diameter (Part 3);

FIG. 18 shows an optical waveform of a vertical cavity surface emittinglaser device having a fundamental transverse-mode optical confinementcoefficient of about 0.983 at 25° C.;

FIG. 19 shows an optical waveform of a vertical cavity surface emittinglaser device having a fundamental transverse-mode optical confinementcoefficient of about 0.846 at 25° C.;

FIG. 20 shows the relationship between thickness of the selectiveoxidation layer and a droop rate related to a vertical cavity surfaceemitting laser device at 25° C.;

FIG. 21 is an illustrative diagram of Δλ₀>0;

FIG. 22 is an illustrative diagram of Δλ₀<0;

FIG. 23 shows the relationship between oscillation threshold current andmeasured temperature;

FIG. 24 shows the relationship between the amount of detuning andtemperature at which the lowest threshold current is obtained;

FIG. 25 shows the relationship between droop rate and temperature atwhich the lowest threshold current is obtained (Part 1);

FIG. 26 shows the relationship between droop rate and temperature atwhich the lowest threshold current is obtained (Part 2);

FIG. 27 shows the relationship between the number of pairs in an opticalconfinement reducing region A and the fundamental transverse-modeoptical confinement coefficient;

FIG. 28 shows the structure of a conventional cavity surface emittinglaser device used for calculation;

FIG. 29 shows the structure of a cavity surface emitting laser devicehaving an optical confinement reducing region used for the calculation;

FIG. 30 shows the structure of a lower semiconductor DBR of theconventional vertical cavity surface emitting laser device used for thecalculation;

FIG. 31 shows the optical confinement reducing region A;

FIG. 32 shows an optical confinement reducing region B;

FIG. 33 shows the relationship between the number of pairs in theoptical confinement reducing region B and the fundamentaltransverse-mode optical confinement coefficient;

FIG. 34 shows an optical confinement reducing region C;

FIG. 35 shows the relationship between the number of pairs in theoptical confinement reducing region C and the fundamentaltransverse-mode optical confinement coefficient;

FIG. 36 is a diagram showing an effect of the optical confinementreducing region;

FIG. 37 is a diagram illustrating an absorption loss reducing layer(Part 1);

FIG. 38 is a diagram illustrating the absorption loss reducing layer(Part 2);

FIG. 39 is a diagram illustrating an influence of the absorption lossreducing layer on the optical confinement coefficient;

FIG. 40 is a diagram illustrating effects of the optical confinementreducing region and the absorption loss reducing layer (Part 1);

FIG. 41 is a diagram illustrating effects of the optical confinementreducing region and the absorption loss reducing layer (Part 2);

FIG. 42 illustrates a first modification of the optical confinementreducing region;

FIG. 43 illustrates a second modification of the optical confinementreducing region;

FIG. 44 illustrates a third modification of the optical confinementreducing region;

FIG. 45 shows a vertical cavity surface emitting laser array;

FIG. 46 shows a two-dimensional array of light-emitting parts of FIG.45;

FIG. 47 is a cross-sectional view along A-A line of FIG. 46;

FIG. 48 is a schematic structure of a color printer;

FIG. 49 illustrates an optical waveform of the conventional verticalcavity surface emitting laser device;

FIG. 50 shows an enlarged view of a rise and its vicinity in the opticalwaveform of FIG. 49.

FIG. 51 is an enlarged view showing a part of the lower semiconductorDBR;

FIG. 52 shows the lower semiconductor DBR of Example 1;

FIG. 53 shows the lower semiconductor DBR of Example 2;

FIG. 54 shows the lower semiconductor DBR in which a third lowersemiconductor DBR includes three pairs of refractive index layers;

FIG. 55 shows calculated results of heat resistance;

FIG. 56 shows a modification of the vertical cavity surface emittinglaser device;

FIG. 57 shows an enlarged view of a part of the lower semiconductor DBR;

FIG. 58 is an enlarged view showing the vicinity of the active layer;

FIG. 59 shows a schematic structure of an optical transmission moduleand an optical transmission system;

FIG. 60 shows a vertical cavity surface emitting laser array included ina light source;

FIG. 61 shows a sectional view along A-A line shown in FIG. 60;

FIG. 62 shows an enlarged view of a part of the lower semiconductor DBRof FIG. 61;

FIG. 63 is an enlarged view showing the vicinity of the active layer ofFIG. 61;

FIG. 64 shows an optical fiber cable of FIG. 59; and

FIG. 65 shows a relationship between an Al component and thermalconductivity of an AlGaAs material.

BEST MODE OF CARRYING OUT THE INVENTION a. First Embodiment

FIG. 49 shows an optical waveform obtained when a vertical cavitysurface emitting laser device is driven under pulse conditions of apulse width of 500 μs and a duty of 50% (pulse period: 1 ms). As shownin FIG. 49, after reaching a peak immediately after the rise time, theoptical output falls off and becomes steady when seen over a relativelylong period of time. The change in the optical output is due toself-heating of the vertical cavity surface emitting laser device, andis in general referred to as “droop characteristic”.

In an in-depth examination conducted by the inventors of the presentinvention, a new finding has been made that changes in the opticaloutput different from the “droop characteristic” occur over a shortperiod of time, as shown in FIG. 50 which provides an enlarged view ofthe rise and its vicinity in the optical waveform of FIG. 49.

According to FIG. 50, the optical output has yet to fully rise after 10ns. The optical output substantially fully rises after about 200 ns, andsubsequently increases gradually until about 1 μs. This phenomenon(characteristic) is a new finding made by the inventors of the presentinvention. In this specification, such a characteristic is referred toas “negative droop characteristic”. Note that the negative droopcharacteristic is not found in conventional edge emitting semiconductorlaser devices.

In order to obtain high image quality with a vertical cavity surfaceemitting laser device, the optical response waveform during the risetime needs to be appropriately controlled, and it has been made clearthat it is difficult to obtain high-quality images using vertical cavitysurface emitting laser devices having the negative droop characteristic.

Next is described one embodiment of the present invention with referenceto FIGS. 1 through 41. FIG. 1 shows a general structure of a laserprinter 1000 according to one embodiment of the present invention.

The laser printer 1000 includes, for example, an optical scanningapparatus 1010, a photoreceptor drum 1030, a charger 1031, a developingroller 1032, a transfer charger 1033, a neutralizing unit 1034, acleaning unit 1035, a toner cartridge 1036, a sheet feeding roller 1037,a sheet feed tray 1038, paired resist rollers 1039, fixing rollers 1041,sheet discharge rollers 1042, a catch tray 1043, a communication controlunit 1050, and a printer control unit 1060 for exercising overallcontrol over the aforementioned components. All of these components aredisposed at predetermined positions in a printer chassis 1044.

The communication control unit 1050 controls bidirectionalcommunications with a higher-level apparatus (e.g. a personal computer),which is connected to the laser printer 1000 via a network.

The photoreceptor drum 1030 has a cylindrical body, on the surface ofwhich a photosensitive layer is formed. That is, the surface of thephotoreceptor drum 1030 is a surface on which scanning is performed. Thephotoreceptor drum 1030 is designed to rotate in a direction indicatedby the arrow in FIG. 1.

The charger 1031, the developing roller 1032, the transfer charger 1033,the neutralizing unit 1034 and the cleaning unit 1035 are disposedadjacent to the surface of the photoreceptor drum 1030. Specifically,these components are disposed along the rotational direction of thephotoreceptor drum 1030 in the stated order.

The charger 1031 uniformly charges the surface of the photoreceptor drum1030.

The optical scanning apparatus 1010 emits, onto the surface of thephotoreceptor drum 1030 which is charged by the charger 1031, a beam oflight modulated based on image information sent from a higher-levelapparatus. Accordingly, a latent image corresponding to the imageinformation is formed on the surface of the photoreceptor drum 1030. Thelatent image is then moved toward the developing roller 1032 as thephotoreceptor drum 1030 rotates. Note that the structure of the opticalscanning apparatus 1010 is described later.

The toner cartridge 1036 houses toner, which is to be supplied to thedeveloping roller 1032.

The developing roller 1032 applies toner supplied from the tonercartridge 1036 to the latent image formed on the surface of thephotoreceptor drum 1030 so as to develop the latent image into a visibleimage. Then, the visible image with toner (hereinafter, also referred toas “toner image” for convenience) is moved toward the transfer charger1033 as the photoreceptor drum 1030 rotates.

The sheet feed tray 1038 houses recording sheets 1040. The sheet feedingroller 1037 is provided near the sheet feed tray 1038. The sheet feedingroller 1037 takes out one recording sheet 1040 at a time from the sheetfeed tray 1038 and conveys it to the paired resist rollers 1039. Theresist rollers 1039 first hold the recording sheet 1040 taken out by thesheet feeding roller 1037, and then send the recording sheet 1040 out tothe gap between the photoreceptor drum 1030 and the transfer charger1033 in accordance with the rotation of the photoreceptor drum 1030.

A voltage having a polarity opposite to that of the toner on the surfaceof the photoreceptor drum 1030 is applied to the transfer charger 1033in order to electrically attract the toner. By the voltage, the tonerimage on the photoreceptor drum 1030 is transferred to the recordingsheet 1040. The recording sheet 1040 onto which the toner image has beentransferred is sent to the fixing rollers 1041.

The fixing rollers 1041 apply heat and pressure to the recording sheet1040, whereby the toner is fixed onto the recording sheet 1040. Then,the recording sheet 1040 on which the toner has been fixed is sent tothe catch tray 1043 via the sheet discharge rollers 1042. Multiplerecording sheets 1040 subjected to such processing procedures aresequentially stacked on the catch tray 1043.

The neutralizing unit 1034 renders the surface of the photoreceptor drum1030 electrically neutral.

The cleaning unit 1035 removes toner remaining (residual toner) on thesurface of the photoreceptor drum 1030. The surface of the photoreceptordrum 1030 from which the residual toner has been removed returns to aposition opposing the charger 1031.

Next is described the structure of the optical scanning apparatus 1010.

As an example as shown in FIG. 2, the optical scanning apparatus 1010includes a deflector-side scanning lens 11 a, an image plane-sidescanning lens 11 b, a polygon mirror 13, a light source 14, a couplinglens 15, an aperture plate 16, an anamorphic lens 17, a reflector mirror18, a scan controller (not shown) and the like. These components aredisposed and fixed at predetermined positions in a housing 30.

Note that a direction corresponding to the main scanning direction and adirection corresponding to the sub-scanning direction are hereinaftersimply referred to as “main scanning corresponding direction” and“sub-scanning corresponding direction”, respectively.

The coupling lens 15 converts a light beam emitted from the light source14 into substantially parallel light.

The aperture plate 16 has an aperture and defines the beam diameter ofthe light passing through the coupling lens 15.

The anamorphic lens 17 converts the beam of light having passed throughthe aperture of the aperture plate 16 so that the beam of light forms,via the reflector mirror 18, an image in the sub-scanning correspondingdirection near the deflecting reflection surfaces of the polygon mirror13.

An optical system disposed in the light path between the light source 14and the polygon mirror 13 may be referred to as a pre-deflector opticalsystem. In the present embodiment, the pre-deflector optical systemincludes the coupling lens 15, the aperture plate 16, the anamorphiclens 17 and the reflector mirror 18.

The polygon mirror 13 includes, for example, a six-faceted mirror whosediameter of an inscribed circle is 18 mm. Each facet of the polygonmirror 13 is a deflecting reflection surface. The polygon mirror 13deflects the beam of light reflected by the reflector mirror 18 as itrotates around an axis parallel to the sub-scanning correspondingdirection at a uniform velocity.

The deflector-side scanning lens 11 a is disposed in the light path ofthe beam of light deflected by the polygon mirror 13.

The image plane-side scanning lens 11 b is disposed in the light path ofthe beam of light having passed through the deflector-side scanning lens11 a. The beam of light having passed through the image plane-sidescanning lens 11 b is projected on the surface of the photoreceptor drum1030, whereby an optical spot is formed. The optical spot shifts in thelongitudinal direction of the photoreceptor drum 1030 as the polygonmirror 13 rotates. That is, the optical spot scans across thephotoreceptor drum 1030. The direction in which the optical spot movesis the “main scanning direction”. On the other hand, the rotationaldirection of the photoreceptor drum 1030 is the “sub-scanningdirection”.

An optical system disposed in the light path between the polygon mirror13 and the photoreceptor drum 1030 may be referred to as a scanningoptical system. In the present embodiment, the scanning optical systemincludes the deflector-side scanning lens 11 a and the image plane-sidescanning lens 11 b. Note that at least one light-path bending mirror maybe disposed in at least one of the light path between the deflector-sidescanning lens 11 a and the image plane-side scanning lens 11 b and thelight path between the image plane-side scanning lens 11 b and thephotoreceptor drum 1030.

The light source 14 includes a vertical cavity surface emitting laserdevice 100, an example of which is shown in FIG. 3. In thisspecification, the laser oscillation direction is referred to as the Zdirection, and two directions mutually orthogonal to each other in aplane perpendicular to the Z direction are referred to as the X and Ydirections.

The vertical cavity surface emitting laser device 100 is designed tohave an oscillation wavelength of 780 nm band, and includes a substrate101, a buffer layer 102, a lower semiconductor DBR (distribution Braggreflector) 103, a lower spacer layer 104, an active layer 105, an upperspacer layer 106, an upper semiconductor DBR 107 and a contact layer109.

The substrate 101 included in the vertical cavity surface emitting laserdevice 100 has a mirror-polished surface. The substrate 101 is an n-GaAsmonocrystalline substrate in which the normal direction of themirror-polished surface is inclined by 15 degrees (θ=15 degrees) from acrystal orientation [1 0 0] toward a crystal orientation [1 1 1]A, asshown in FIG. 4A. That is to say, the substrate 101 is an inclinedsubstrate. In this embodiment, the substrate 101 is disposed in such amanner that a crystal orientation [0 1 −1] is aligned in the +Xdirection and a crystal orientation [0 −1 1] is aligned in the −Xdirection, as shown in FIG. 4B.

The buffer layer 102 is an n-GaAs layer laid on a +Z-direction surfaceof the substrate 101.

The lower semiconductor DBR 103 includes a first lower semiconductor DBR103 ₁, a second lower semiconductor DBR 103 ₂ and a third lowersemiconductor DBR 103 ₃, of which an example is shown in FIG. 5.

The first lower semiconductor DBR 103 ₁ is laid over a +Z-directionsurface of the buffer layer 102. The first lower semiconductor DBR 103 ₁includes 36.5 pairs of an n-AlAs low refractive index layer 103 a and ann-Al_(0.3)Ga_(0.7)As high refractive index layer 103 b. In order toreduce electrical resistance, a compositionally graded layer (not shown)is provided between each two neighboring refractive index layers. In thecompositionally graded layer, the composition is gradually changed fromone to another. It is designed that each refractive index layer has anoptical thickness of λ/4 (where λ is an oscillation wavelength) byincluding ½ the thickness of its neighboring compositionally gradedlayer. When the optical thickness is λ/4, the actual thickness d of thelayer is λ/4N (where N is a refractive index of the material of thelayer).

The second lower semiconductor DBR 103 ₂ is laid on a +Z-directionsurface of the first lower semiconductor DBR 103 ₁, and includes threepairs of the low refractive index layer 103 a and the high refractiveindex layer 103 b. In order to reduce electrical resistance, acompositionally graded layer (not shown) is provided between each twoneighboring refractive index layers. It is designed that each lowrefractive index layer 103 a has an optical thickness of 3λ/4 byincluding ½ the thickness of its neighboring compositionally gradedlayer, and each high refractive index layer 103 b has an opticalthickness of λ/4 by including ½ the thickness of its neighboringcompositionally graded layer. The second lower semiconductor DBR 103 ₂is an “optical confinement reducing region”.

The third lower semiconductor DBR 103 ₃ is laid on a +Z-directionsurface of the second lower semiconductor DBR 103 ₂, and includes a pairof the low refractive index layer 103 a and the high refractive indexlayer 103 b. In order to reduce electrical resistance, a compositionallygraded layer (not shown) is provided between each two neighboringrefractive index layers. It is designed that each refractive index layerhas an optical thickness of λ/4 by including ½ the thickness of itsneighboring compositionally graded layer.

Thus, in the present embodiment, the lower semiconductor DBR 103includes 40.5 pairs of the low and high refractive index layers 103 aand 103 b.

The lower spacer layer 104, which is a non-doped(Al_(0.1)Ga_(0.9))_(0.5)In_(0.5)P layer, is laid on a +Z-directionsurface of the third lower semiconductor DBR 103 ₃.

The active layer 105 is laid on a +Z-direction surface of the lowerspacer layer 104. The active layer 105 is a threefold quantum wellactive layer including GaInAsP quantum well layers 105 a and GaInPbarrier layers 105 b, as an example as shown in FIG. 6. Each quantumwell layer 105 a is created by introducing As into a GaInp mixed crystalin order to obtain a 780 nm-band oscillation wavelength, and has acompression strain. The barrier layers 105 b have large band gaps withthe introduction of tensile strain, thereby providing high carrierconfinement, and also function as a strain-compensation structure forthe quantum well layers 105 a.

In this embodiment, since an inclined substrate is used as the substrate101, anisotropy is introduced into the gain of the active layer, therebymaking possible to align the direction of polarization in apredetermined direction (polarization control).

The upper spacer layer 106, which is a non-doped(Al_(0.1)Ga_(0.9))_(0.5)In_(0.5)P layer, is laid on a +Z-directionsurface of the active layer 105.

A section including the lower spacer layer 104, the active layer 105 andthe upper spacer layer 106 is referred to as a resonator structure,which is designed to have an optical thickness of λ. The PL wavelengthof the active layer 105 is designed to be 772 nm, which is 8 nm shorterthan the resonance wavelength, 780 nm, of the resonator structure, andthe lowest threshold current is obtained at 17° C. The active layer 105is provided in the center of the resonator structure, which correspondsto an antinode of the standing wave of the electric field, in order toachieve a high stimulated emission rate. The resonator structure issandwiched between the lower semiconductor DBR 103 and the uppersemiconductor DBR 107.

The upper semiconductor DBR 107 includes a first upper semiconductor DBR107 ₁ and a second upper semiconductor DBR 107 ₂.

The first upper semiconductor DBR 107 ₁ is laid on a +Z-directionsurface of the upper spacer layer 106, and includes one pair of ap-(Al_(0.7)Ga_(0.3))_(0.5)In_(0.5)P low refractive index layer and ap-(Al_(0.1)Ga_(0.9))_(0.5)In_(0.5)P high refractive index layer. Inorder to reduce electrical resistance, a compositionally graded layer isprovided between each two neighboring refractive index layers. It isdesigned that each refractive index layer has an optical thickness ofλ/4 by including ½ the thickness of its neighboring compositionallygraded layer.

The first upper semiconductor DBR 107 ₁ has higher band gap energycompared to an AlGaAs layer, and functions as a blocking layer forblocking electrons injected into an active region.

Since an inclined substrate is used as the substrate 101, it is possiblenot only to reduce the occurrence of the hillock defect formation of theAlGaInP material and improve the crystallinity, but also to reduce theoccurrence of natural superlattice and prevent a decrease in band gapenergy. Accordingly, the first upper semiconductor DBR 107 ₁ is able tomaintain high band gap energy, and favorably functions as an electronblocking layer.

The second upper semiconductor DBR 107 ₂ is laid on a +Z-directionsurface of the first upper semiconductor DBR 107 ₁, and includes 23pairs of a p-Al_(0.9)Ga_(0.1)As low refractive index layer and ap-Al_(0.3)Ga_(0.7)As high refractive index layer. In order to reduceelectrical resistance, a compositionally graded layer is providedbetween each two neighboring refractive index layers. It is designedthat each refractive index layer has an optical thickness of λ/4 byincluding ½ the thickness of its neighboring compositionally gradedlayer.

In one low refractive index layer of the second upper semiconductor DBR107 ₂, a p-AlAs selective oxidation layer having a thickness of 30 nm isinserted. The selective oxidation layer is provided, within the lowrefractive index layer of the third pair from the upper spacer layer106, at a position corresponding to nodes of the standing wave of theelectric field.

The contact layer 109 is a p-GaAs layer laid on a +Z-direction surfaceof the second upper semiconductor DBR 107 ₂.

A resultant structure in which multiple semiconductor layers are laidover the substrate 101 is hereinafter also referred to as “laminatedbody”.

In addition, the optical thickness of each refractive index layerdescribed below includes ½ the thickness of its neighboringcompositionally graded layer.

Next is a brief description of a method for manufacturing the verticalcavity surface emitting laser device 100.

(1) The above-described laminated body is created by a crystal growthmethod, such as metal-organic chemical vapor deposition (MOCVD method)or molecular beam epitaxy (MBE method).

In this step, trimethylaluminium (TMA), trimethyl gallium (TMG) andtrimethyl indium (TMI) are used as the group-III materials, and arsineaddition, carbon tetrabromide (CBr₄) is used as a p-type dopant, andhydrogen selenide (H₂Se) is used as an n-type dopant. Phosphine (PH₃)gas is used as the group-V P material of the AlGaInAsP material, anddimethylzinc (DMZn) is used as a p-type dopant of AlGaInP.

(2) A square resist pattern, each side of which is 25 μm, is formed onthe surface of the laminated body.

(3) Using the square resist pattern as a photomask, a square columnarmesa is formed by ECR etching using Cl₂ gas. The etching bottom ispositioned in the lower semiconductor DBR 103.

(4) The photomask is removed.

(5) The laminated body is heat-treated in water vapor. In this step, Alin the selective oxidation layer is selectively oxidized from theperiphery of the mesa. Accordingly, an unoxidized region 108 a which issurrounded by an AL oxidized layer 108 b is left in the center of themesa (see FIG. 3). In this manner, an oxidized current confinementstructure is formed, in which a pathway of the current for driving alight-emitting part is limited to the center of the mesa. The unoxidizedregion 108 a functions as a current passage region (current injectionregion). Appropriate conditions of the heat treatment (holdingtemperature, holding time and the like) are selected based on results ofvarious preliminary experiments so that each side of the current passageregion is about 4 μm. Specifically, the holding temperature is 360° C.and the holding time is 30 minutes.

(6) A protective layer 111 made of SiN or SiO₂ is formed by chemicalvapor deposition (CVD method).

(7) Polyimide 112 is used to planarize the laminated body.

(8) Apertures for p-electrode contact are provided on the top of themesa. In this step, a photoresist mask is provided on the top of themesa, and then, locations on the mesa, at which the apertures are to beformed, are exposed to light to remove the photoresist mask from thelocations. Subsequently, the apertures are formed by buffered HF etching(BHF etching) the polyimide 112 and the protective layer 111.

(9) A square resist pattern, each side of which is 10 μm, is formed onthe top of the mesa at a region to be a light emitting part, andp-electrode materials are then deposited by vapor-deposition. As thep-electrode materials, a multilayer film made of Cr/AuZn/Au or Ti/Pt/Auis used.

(10) The p-electrode materials are lifted off from the region to be thelight emitting part, whereby a p-electrode 113 is formed.

(11) The back side of the substrate 101 is polished so that thesubstrate 101 has a predetermined thickness (about 100 μm, for example),and then, an n-electrode 114 is formed. The n-electrode 114 is amultilayer film made of AuGe/Ni/Au.

(12) The p-electrode 113 and the n-electrode 114 are ohmically connectedby annealing, whereby the mesa becomes a light-emitting part.

(13) The laminated body is cut into chips.

An examination was conducted by applying a square wave current pulsehaving a pulse period of 1 ms and a pulse width of 500 μs (a duty of50%) to the vertical cavity surface emitting laser device 100manufactured in the above-described manner, with a target of producingan optical output of 1.4 mW. The result was (P1−P2)/P2=−0.06, where P1is the optical output obtained 10 ns after the application and P2 is theoptical output obtained 1 μs after the application. Note that a valueobtained from the formula (P1−P2)/P2×100 (unit: %) is also referred toas “droop rate” below. Thus, the droop rate of the vertical cavitysurface emitting laser device 100 of the present embodiment is −6%. Itshould be noted that if a vertical cavity surface emitting laser devicehaving a droop rate of less than −10% is used in a laser printer, animage output from the laser printer is highly likely to have a blurredcontour, at least partially, when viewed by the naked eye.

In the above examination, the vertical cavity surface emitting laserdevice 100 produced a single fundamental transverse-mode output of morethan 2 mW.

In addition, the vertical cavity surface emitting laser device 100exhibited threshold current characteristics and an external differentialquantum efficiency (slope efficiency) equivalent to those ofconventional vertical cavity surface emitting laser devices.

The inventors of the present invention examined in detail opticalwaveforms obtained when a conventional vertical cavity surface emittinglaser device having an oxidized current confinement structure was drivenby various different square wave current pulses. FIG. 7 shows an opticalwaveform obtained with a pulse period of 1 ms and a duty of 50%, andFIG. 8 shows an optical waveform obtained with a pulse period of 100 nsand a duty of 50%.

According to the optical waveform of FIG. 7, the negative droopcharacteristic is shown in which the optical output gradually increasesafter the rise time. Even after 60 ns, the optical output does not reachthe target value (1.5 mW). On the other hand, according to the opticalwaveform of FIG. 8, the optical output after the rise time is stable andthe negative droop characteristic does not appear.

Accordingly, it is understood that, even if square wave current pulsesapplied to the conventional vertical cavity surface emitting laserdevice have the same duty, i.e. the same heating value, the negativedroop characteristic appears if the applied square wave current pulsehas a long pulse period and the negative droop characteristic does notappear if the applied square wave current pulse has a short pulseperiod.

It is contemplated that the difference in the pulse period leads to adifference in the internal temperature of the vertical cavity surfaceemitting laser device. That is, in the case of a long pulse period, theheating periods and the cooling-down periods are both long, the internaltemperature of the vertical cavity surface emitting laser device largelychanges. On the other hand, in the case of a short pulse period,cooling-down periods do not last long. Therefore, changes in theinternal temperature of the vertical cavity surface emitting laserdevice are small, and the internal temperature remains relatively highon average. That is to say, with the driving conditions causing thenegative droop characteristic, the internal temperature of the verticalcavity surface emitting laser device largely changes, and thus, it isdeduced that the negative droop characteristic is a phenomenonattributable to the internal temperature of the vertical cavity surfaceemitting laser device.

If the internal temperature of the vertical cavity surface emittinglaser device changes, an electric field intensity distribution of theoscillation modes in the transverse direction (hereinafter, referred toalso as “transverse-mode distribution” below) also changes.

The oxidized layer in the oxidized current confinement structure has arefractive index of about 1.6, which is lower than that of theneighboring semiconductor layers (about 3). Accordingly, inside thevertical cavity surface emitting laser device, a so-called built-ineffective refractive index difference Δneff is present in the transversedirection (see FIG. 9).

By the effective refractive index difference Δneff, oscillation modesincluding the fundamental transverse mode are confined in the transversedirection. At this point, the spread of the oscillation modes in thetransverse direction depends on the size of Δneff, and the larger Δneff,the smaller the spread in the transverse direction (see FIGS. 10A and10B).

If a current (drive current) is injected into the vertical cavitysurface emitting laser device, the current is concentrated in thecentral portion of the mesa (hereinafter, “mesa central portion”). Then,due to Joule heat, nonradiative recombination in the active layer regionand the like, particularly a part of the mesa central portion close tothe active layer has a higher local temperature compared to thesurrounding region. If the temperature of a semiconductor material isincreased, the semiconductor material has reduced band gap energy, whichleads to a high refractive index. Therefore, if the local temperature ofthe mesa central portion is increased, the refractive index of the mesacentral portion becomes higher compared to that of the surroundingregion, and accordingly, the optical confinement in the transversedirection becomes significant.

As shown in FIG. 10A, in the case where the built-in effectiverefractive index difference Δneff is small, if the local temperature ofthe mesa central portion is increased, a change in the effectiverefractive index difference Δneff becomes large, as shown in FIG. 11A,which results in a large change in the transverse-mode distribution. Inthis case, the overlap between the gain region into which a current isbeing injected and the transverse mode increases, and the opticalconfinement in the transverse direction becomes significant. As aresult, the light intensity in emission rate increases, and thethreshold current is accordingly reduced.

Thus, as to a vertical cavity surface emitting laser device having asmall built-in effective refractive index difference Δneff and havinginsufficient optical confinement in the transverse direction at roomtemperature, if the internal temperature increases, an I-L curve(injection current-optical output curves) is wholly shifted toward thelower current side, and the luminous efficiency is improved (see FIG.12). In this case, the optical output obtained with the same drivecurrent value increases over time, and thus, the negative droopcharacteristic is observed (see FIG. 13). FIG. 12 shows an estimated I-Lcharacteristic for a time t=t₀ sec, which is prior to the increase ofthe internal temperature, and an estimated I-L characteristic for a timet=t₁ sec at which the internal temperature has sufficiently increasedwith the supply of a pulsed drive current. Along with the increase ofthe temperature, the luminous efficiency is improved and the thresholdcurrent is reduced, and therefore, the I-L characteristic of t₁ sec isshifted towards the Since the drive current is constant at I_(op), theoptical output is larger in the case of t₁ sec. The optical waveform ofthis case is shown in FIG. 13.

On the other hand, in the case where the built-in effective refractiveindex difference Δneff is large, as shown in FIG. 10B, even if the localtemperature of the mesa central portion is increased, a change in theeffective refractive index difference Δneff is small, as shown in FIG.11B. Accordingly, little change is observed in the transverse-modedistribution.

Thus, as to a vertical cavity surface emitting laser device having alarge built-in effective refractive index difference Δneff and havingsufficient optical confinement in the transverse direction at roomtemperature, even if the internal temperature increases, thetransverse-mode distribution is stable and little change is seen in theluminous efficiency. In this case, the optical output obtained with thesame drive current value remains substantially constant over time, andthus, the negative droop characteristic does not appear.

A transverse-direction optical confinement coefficient (hereinafter,referred to simply as “optical confinement coefficient”) is used as anindex indicating strength of the optical confinement in the transversedirection. Note that the optical confinement coefficient can be obtainedfrom the ratio of “an integrated intensity of an electric field locatedwithin the radius range in which the current passage region is located”to “an integrated intensity of an electric field on the X-Y crosssection passing through the center of the vertical cavity surfaceemitting laser device”. The larger the optical confinement coefficient,the more the distribution of the electric field intensity acutelyconcentrates on the gain region. In other words, the larger the opticalconfinement coefficient obtained at room temperature, the moresufficiently the optical confinement is achieved by the oxidized currentconfinement structure, which indicates that the transverse modedistribution is stable even during a local temperature change of thegain region.

The transverse-mode distribution of the vertical cavity surface emittinglaser device can be estimated by calculating the distribution of theelectric field intensity using the following Helmholtz equations(Equations (1) and (2)).

$\begin{matrix}\left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack & \; \\{{\left( {\frac{\partial^{2}}{\partial x^{2}} + \frac{\partial^{2}}{\partial y^{2}} + {k_{0}^{2}\left( {{ɛ\left( {x,y} \right)} - n_{{eff},m}^{2}} \right)}} \right){E_{m}\left( {x,y,z} \right)}} = 0} & (1) \\\left\lbrack {{Equation}\mspace{14mu} 2} \right\rbrack & \; \\{{E_{m}\left( {x,y,z} \right)} = {{E_{m}\left( {x,y} \right)}{\exp \left( {\; k_{0}n_{{eff},m}z} \right)}}} & (2)\end{matrix}$

Note however that equations (1) and (2) are analytically difficult tocalculate, and therefore, a numerical analysis using the finite elementtechnique with a calculator is generally performed. Various tools can beused as a solver for the finite element technique, and acommercially-produced VCSEL simulator (e.g. LASER MOD) is an example ofsuch.

A fundamental transverse-mode distribution of a 780 nm-band verticalcavity surface emitting laser device is calculated as an example.

In the vertical cavity surface emitting laser device used for thecalculation, the active layer has a threefold quantum well structureincluding Al_(0.12)Ga_(0.88)As layers (each having a thickness of 8 nm)and Al_(0.3)Ga_(0.7)As layers (each having a thickness of 8 nm). Eachspacer layer is made of Al_(0.6)Ga_(0.4)As. The lower semiconductor DBRincludes 40.5 pairs of an Al_(0.3)Ga_(0.7)As high refractive index layerand an AlAs low refractive index layer. The upper semiconductor DBRincludes 24 pairs of an Al_(0.3)Ga_(0.7)As high refractive index layerand an Al_(0.9)Ga_(0.1)As low refractive index layer.

The vertical cavity surface emitting laser device has a cylindrical mesahaving a diameter of 25 μm. The mesa etching has been carried out up tothe boundary between the lower semiconductor DBR and the lower spacerlayer, and the etched regions are filled with atmospheric air. That is,the vertical cavity surface emitting laser device has a simple etchedmesa structure. The lower semiconductor DBR, which is not subjected tomesa etching, has a diameter of 35 μm, which is the maximum widthconcerned in the calculation. The selective oxidation layer made of AlAsis disposed within the low refractive index layer having an opticalthickness of 3λ/4 in the upper semiconductor DBR, and more specifically,disposed at a position corresponding to the third node of the standingwave counted from the active layer.

Note that the calculation does not take into account the gain of theactive layer and the absorption by the semiconductor material, andobtains only an eigenmode distribution determined by the structure. Thetemperature of the vertical cavity surface emitting laser device is keptconstant at 300 K. The refractive index of each material is as shown inFIG. 14. Note that the oxidized layer of the oxidized currentconfinement structure is also referred to simply as “oxidized layer”,and the diameter of the current passage region is also referred to as“oxide confinement diameter”.

Based on a fundamental transverse mode distribution calculated in theabove-described manner, an optical confinement coefficient Γ₁ iscalculated using the following equation (3). In the equation, a is theradius of the current passage region.

$\begin{matrix}\left\lbrack {{Equation}\mspace{14mu} 3} \right\rbrack & \; \\{\Gamma_{l} = \frac{\int_{0}^{a}{{E}^{2}\ {r}}}{\int_{0}^{\infty}{{E}^{2}\ {r}}}} & (3)\end{matrix}$

The fundamental transverse-mode optical confinement coefficient of the780 nm-band vertical cavity surface emitting laser device at roomtemperature is calculated for various thicknesses of the selectiveoxidation layer and various oxide confinement diameters. The calculationresults are shown in FIG. 15. According to the results, the opticalconfinement coefficient depends on the thickness of the selectiveoxidation layer and the oxide confinement diameter, and the greater thethickness of the selective oxidation layer and the larger the oxideconfinement diameter, the higher the optical confinement coefficient.

FIG. 16 is a graph illustrating the calculation results of FIG. 15 withthe thickness of the selective oxidation layer on the horizontal axisand the optical confinement coefficient on the vertical axis. As forchange in the optical confinement coefficient associated with anincrease in the thickness of the selective oxidation layer, it can beseen for all the different oxide confinement diameters that the changeis significant when the thickness of the selective oxidation layer is 25nm or less and the change tends to be saturated when the thickness ofthe selective oxidation layer is 25 nm or more.

Multiple vertical cavity surface emitting laser devices having variousthicknesses of the selective oxidation layer and various oxideconfinement diameters were manufactured in order to evaluate the droopcharacteristic of them. FIG. 17 shows the evaluation results. In FIG.17, “∘” denotes a droop rate of −10% or more and “x” denotes a drooprate of less than −10%. According to FIGS. 15 and 17, it is understoodthat device structures having a fundamental transverse-mode opticalconfinement coefficient of 0.9 or more at room temperature exhibit adroop rate of −10% or more.

FIG. 18 shows an optical waveform of the vertical cavity surfaceemitting laser device having a fundamental transverse-mode opticalconfinement coefficient of about 0.983 at room temperature. The drooprate of this optical waveform is about −4.3%.

FIG. 19 shows an optical waveform of the vertical cavity surfaceemitting laser device having a fundamental transverse-mode opticalconfinement coefficient of about 0.846 at room temperature. The drooprate of this optical waveform is about −62.8%.

Various vertical cavity surface emitting laser devices having differentoptical confinement coefficients were manufactured, and an in-deptexamination was performed. According to the examination, the droop rateis about −5% if the optical confinement coefficient is about 0.9, and ifthe optical confinement coefficient increases further, the droop rateincreases along with the increase in the optical confinementcoefficient. On the other hand, if the optical confinement coefficientis less than 0.9, the droop rate decreases as the optical confinementcoefficient becomes smaller. In the examination, some vertical cavitysurface emitting laser devices having small optical confinementcoefficients exhibited a droop rate of −70% or less.

Thus, if the fundamental transverse-mode optical confinement coefficientat room temperature is 0.9 or more, the negative droop characteristiccan be suppressed.

In general, the effective refractive index difference Δneff at roomtemperature becomes larger as the selective oxidation layer is larger inthickness and as the selective oxidation layer is disposed closer to theactive layer. Note however that if the degrees of influence of these twofactors are compared, the thickness of the selective oxidation layer hasa greater influence on the effective refractive index difference Δneff.Accordingly, the strength of the optical confinement in the transversedirection is determined chiefly by the thickness of the selectiveoxidation layer.

Commonly used oxide confinement diameters are 4.0 μm and more. As shownin FIG. 15, if the oxide confinement diameter is 4.0 μm or more and thethickness of the selective oxidation layer is 25 nm or more, an opticalconfinement coefficient of 0.9 or more can be obtained.

FIG. 20 shows the relationship between the thickness of the selectiveoxidation layer 108 and the droop rate of a vertical cavity surfaceemitting laser device having a square columnar mesa and an oxideconfinement diameter of 4 μm or more. The droop rate of FIG. 20 iscalculated from an optical waveform obtained when the vertical cavitysurface emitting laser device is driven by applying a square wavecurrent pulse having a pulse period of 1 ms and a duty of 50%. Accordingto FIG. 20, if the thickness of the selective oxidation layer decreases,the droop rate decreases in an exponential manner, and the negativedroop characteristic becomes prominent. Also, variation in the drooprate among the vertical cavity surface emitting laser devices becomesprominent. In order to have a droop rate of −10% or more, the selectiveoxidation layer should be 25 nm or more in thickness.

Since the fundamental transverse-mode optical confinement coefficientdepends mainly on both the oxide confinement diameter and the thicknessof the selective oxidation layer, it is important how to select thecombination of the oxide confinement diameter and the thickness of theselective oxidation layer.

The inventors experimented with various fitting techniques for thecombination of the oxide confinement diameter and the thickness of theselective oxidation layer. As a result, it was found that thecalculation results of FIG. 15 can be largely expressed by a quadraticform with two variables, the oxide confinement diameter (d [μm]) and thethickness of the selective oxidation layer (t [nm]). The followingEquation (4) is obtained by fitting the fundamental transverse-modeoptical confinement coefficient (Γ) into the quadratic form with theoxide confinement diameter (d) and the thickness of the selectiveoxidation layer (t). By assigning specific values of FIG. 15 into d andt, the fundamental transverse-mode optical confinement coefficient ofFIG. 15 can be obtained with an error of roughly 1%.

Γ(d,t)=−2.54d ²−0.14t ²−0.998d·t+53.4d+12.9t−216  [Equation 4]

As described above, in order to effectively suppress the negative droopcharacteristic, the optical confinement coefficient needs to be 0.9 ormore. The combination (range) of the oxide confinement diameter (d) andthe thickness of the selective oxidation layer (t) for obtaining anoptical confinement coefficient of 0.9 or more can be found by the aboveEquation (4). That is, the range represents combinations of d and t thatsatisfy an inequality of Γ(d, t)≧0.9, and more specifically, the rangeis expressed by the next Equation (5).

−2.54d ²−0.14t ²−0.998d·t+53.4d+12.9t−216≧0.9  [Equation 5]

By selecting the oxide confinement diameter (d) and the thickness of theselective oxidation layer (t) so as to satisfy the above Equation (5),it is possible to achieve an optical confinement coefficient of 0.9 ormore, thereby obtaining a vertical cavity surface emitting laser devicefree from negative droop characteristic.

In the past, it had not been known that Δneff has influence on the droopcharacteristic, and the influence has been made known for the first timeby the inventors of the present invention.

In the process of selectively oxidizing Al (the above process of (5)),oxidation progresses not only in the direction parallel to the substrateplane (in this case, X-Y in-plane direction) but also in the verticaldirection (Z direction), if little. Therefore, it can be observed on thecross section of the mesa after the selective oxidation with an electronmicroscope that the oxidized layer is not uniform in thickness, and theperiphery of the mesa (at which oxidation started) is thicker and thethickness of the oxidized layer is less where the oxidation ended(referred to simply as “oxidation end region”). Note however that in theregion extending up to 2 to 3 μm toward the periphery from the oxidationend region, the thickness of the oxidized layer substantiallycorresponds to that of the selective oxidation layer. Since anoscillating light is affected chiefly by the effective refractive indexdifference of the oxidation end region, the selective oxidation layer iscontrolled to have a desired thickness (25 nm or more) in the aboveprocess (1), whereby the oxidation end region in the oxidized layer hasa desired thickness.

A change in the internal temperature of the vertical cavity surfaceemitting laser device results in not only a change in the opticalconfinement coefficient but also a change in the amount of detuning.Next is described the relationship between the amount of detuning andthe negative droop characteristic.

In an edge emitting semiconductor laser device, a laser oscillationoccurs at a gain peak wavelength λg since resonant longitudinal modesare close by. On the other hand, a vertical cavity surface emittinglaser device has a single resonance wavelength in general, and only asingle longitudinal mode is present in the reflection band of thesemiconductor DBR. In addition, since a laser oscillation occurs at aresonance wavelength λr, the emission characteristics of the verticalcavity surface emitting laser device depend on the resonance wavelengthλr and the gain peak wavelength λg of the active layer.

An amount of detuning Δλ₀ is defined by the following Equation (6). λr₀is the resonance wavelength, and λg₀ is the gain peak wavelength. Notethat the suffix “0” indicates that the value is obtained by driving thevertical cavity surface emitting laser device in a CW (Continuous WaveOscillation) mode at the threshold current at room temperature. A valuewithout the suffix “0” below indicates that the value is obtained undera different condition—for example, obtained by driving the verticalcavity surface emitting laser device at a current larger than thethreshold current.

Δλ₀ =Δr ₀ −λg ₀  [Equation 6]

FIG. 21 shows the case of Δλ₀>0, and FIG. 22 shows the case of Δλ₀<0.

The oscillation wavelength is determined not by the gain peakwavelength, but by the resonance wavelength, and therefore, the lasercharacteristics of the vertical cavity surface emitting laser devicedepend largely on whether Δλ₀ is a positive or negative value and themagnitude of the value. For example, the threshold current at roomtemperature tends to increase as the absolute value of Δλ₀ increases.

Along with an increase in the temperature, both the resonance wavelengthand the gain peak wavelength become longer. The change of the resonancewavelength occurs due to a change in the refractive index of thematerials making up the resonator structure, and the change of the gainpeak wavelength occurs due to a change in the band gap energy of thematerials of the active layer. Note that the rate of the change in theband gap energy is greater than the rate of the change in the refractiveindex by about one digit. Accordingly, the emission characteristics at atime when the temperature is changed depend mainly on the amount ofchange in the gain peak wavelength. Note that the resonance wavelengthhas a temperature change rate of about 0.05 nm/K, which is practically anegligible change.

In the vertical cavity surface emitting laser device, if the internaltemperature (the temperature of the active layer) is increased by achange in the injected current or the like, the gain peak wavelengthshifts to the longer wavelength side. In the case of Δλ₀>0 (see FIG.21), the absolute value of Δλ (the degree of detuning) decreases once,and then increases.

In general, a vertical cavity surface emitting laser device has thehighest oscillation efficiency (luminous efficiency) when the gain peakwavelength and the resonance wavelength coincide with each other.

In the case of Δλ₀>0, if the temperature of the device (ambienttemperature) is increased from room temperature, the threshold currentstarts decreasing with the increase in the temperature of the device.Then, the threshold current reaches the minimum value when the gain peakwavelength and the resonance wavelength coincide with each other, andstarts increasing when the temperature is further increased. That is tosay, the temperature at which the lowest threshold current is obtainedis higher than room temperature.

In the case of Δλ₀<0 (see FIG. 22), if the internal temperature (thetemperature of the active layer) is increased, the absolute value of Δλsimply increases. Therefore, if the temperature of the device isincreased from room temperature, the threshold current simply increaseswith the increase in the temperature of the device.

In this case, if the temperature of the device is decreased from roomtemperature, the gain peak wavelength Δλg shifts to the shorterwavelength side. Given this factor, if the temperature of the device isdecreased from room temperature, the threshold current startsdecreasing, and then reaches the minimum value when the gain peakwavelength and the resonance wavelength coincide with each other.Subsequently, if the temperature of the device is further decreased, thethreshold current starts increasing. That is to say, in the case ofΔλ₀<0, the temperature at which the lowest threshold current is obtainedis lower than room temperature.

Using three devices having different Δλ₀ (Δλ₀<0, Δλ₀≈0 and Δλ₀>0), theoscillation threshold current was measured by changing the temperatureof each device (ambient temperature). The measurement results are shownas examples in FIG. 23. The vertical axis of FIG. 23 shows valuesobtained by standardizing an oscillation threshold current at eachtemperature (Ith) through division by an oscillation threshold currentat 25° C. (room temperature) (Ith(25° C.)). The following can be seenfrom FIG. 23: in the case of Δλ₀<0, the threshold current becomes thelowest at a temperature lower than room temperature; in the case ofΔλ₀≈0, the threshold current becomes the lowest at a temperature aroundroom temperature; and in the case of Δλ₀>0, the threshold currentbecomes the lowest at a temperature higher than room temperature).

In order to prevent degradation of the emission characteristics inhigh-temperature and high-power operations, a conventional verticalcavity surface emitting laser device is generally designed to have Δλ₀>0so that the threshold current is reduced at high temperatures.

However, if the conventional vertical cavity surface emitting laserdevice with Δλ₀>0 is driven by a square wave current pulse, the I-Lcurve shifts to the lower current side with an increase in the internaltemperature, and thus the threshold current decreases. Accordingly, theoptical output obtained with a constant drive current value increasesover time. That is, the negative droop characteristic occurs. On theother hand, in the case of Δλ₀<0, the I-L curve shifts to the highercurrent side with an increase in the internal temperature, andtherefore, the optical output does not increase. That is, the negativedroop characteristic does not occur. Thus, in order to suppress thenegative droop characteristic, the following conditions have to be metbesides the thickness of the oxidized layer: Δλ₀<0; and the lowestthreshold current is not obtained at a temperature higher than roomtemperature.

In order to assign a desired value to λ₀, it is necessary to determinethe gain peak wavelength λg₀. In the case of an edge emittingsemiconductor laser device, since the oscillation wavelength coincideswith the gain peak wavelength, the gain peak wavelength can bedetermined from the oscillation wavelength. However, with a verticalcavity surface emitting laser device, the resonance wavelength isdetermined based on its structure. Therefore, it is difficult toestimate the gain peak wavelength, unlike the case of an edge emittingsemiconductor laser device.

Accordingly, either one of the following methods is adopted: (1)manufacturing an edge emitting semiconductor laser device having thesame active layer and estimating the gain peak wavelength from theoscillation wavelength at room temperature; and (2) manufacturing adouble-hetero structure having the same active layer and estimating thegain peak wavelength from the photoluminescence wavelength (PLwavelength).

In the case of adopting the above method (1), for example, anoxide-stripe edge emitting semiconductor laser device having the sameactive layer structure with a stripe width of 40 μm and a resonatorlength of 500 μm is manufactured, and a wavelength of the edge emittingsemiconductor laser device obtained at the threshold current in a CWoperation at room temperature is used as the gain peak wavelength λg₀.

In the case of adopting the above method (2), since the wavelengthduring the laser oscillation shifts in the longitudinal direction(wavelength shift) in relation to the PL wavelength, it is necessary tomake an adjustment. The wavelength shift is attributable to thedifference in the process of the excitation, such as photo-excitationand current excitation, or the influence of heat generated by thecurrent in the case of current excitation. In general, the oscillationwavelength of an edge emitting semiconductor laser device is longer thana PL wavelength APL by about 10 nm. Therefore, assume here that theamount of the wavelength shift is 10 nm.

Accordingly, based on the PL wavelength, the above Equation (6) can beexpressed as the following Equation (7).

Δλ₀ =λr ₀ −λg ₀ =λr ₀−(λPL+10)=λr ₀ −λPL−10  [Equation (7)]

The above amount of the wavelength shift, 10 nm, is a general figure;however, it may be changed according to a material system used.

In an experiment, various vertical cavity surface emitting laser deviceseach having a different Δλ₀ were manufactured, and the temperature atwhich the lowest threshold current was obtained was found for eachvertical cavity surface emitting laser. FIG. 24 shows the experiment'sresults. It can be seen from FIG. 24 that the lowest threshold currentis obtained at room temperature when Δλ₀ is 0.

In the next experiment, various vertical cavity surface emitting laserdevices each having the selective oxidation layer of a differentthickness (30, 31 or 34 nm) were manufactured. Each vertical cavitysurface emitting laser device was driven by changing the optical pulseoutput in order to find the droop rate and the temperature at which thelowest threshold current was obtained. FIGS. 25 and 26 show, accordingto the thickness of the selective oxidation layer, the relationshipsbetween the droop rate and the temperature at which the lowest thresholdcurrent is obtained.

Specifically, FIG. 25 shows the droop rate obtained when the verticalcavity surface emitting laser devices were driven by current pulsesyielding an optical output of 1.4 mW. FIG. 26 shows the droop rateobtained when the vertical cavity surface emitting laser devices, whichare the same as those in FIG. 25, were driven by current pulses yieldingan optical output of 0.3 mW.

When FIGS. 25 and 26 are compared, it can be seen that the droop ratechanges depending on the optical output. With the smaller optical output(i.e. 0.3 mW), the droop rate is lower, and the negative droopcharacteristic appears prominently.

It is considered that in the case of a large optical output, the amountof the injected current is also large and the heating value of thedevice is large, and therefore, the influence of the power saturationdue to heat appears prominently from the beginning of the currentapplication. That is, it is considered that the normal droopcharacteristic appears at a relatively early stage. Note that thenegative droop characteristic is a phenomenon in which the optical pulseoutput gradually increases from the beginning of the current applicationup to 1 μs. Therefore, according to the fact that the influence of thepower saturation due to heat appears at the beginning of the currentapplication, it can be considered that the negative droop characteristichas been improved.

Thus, even using the same devices, the droop rate is changed by changingthe optical output of the devices. The lower the optical output, themore prominently the negative droop characteristic appears.

In printing systems, the optical pulse intensity is modulated in orderto express gray scales of an image. Accordingly, in order to achieve ahigh-definition image, it is very important that the negative droopcharacteristic be suppressed over a wide output range from low to highoutput. As described above, since the negative droop characteristicappears more prominently with a lower output, it is very important tosuppress the negative droop characteristic when the optical output islow. This is an issue newly found by the inventors of the presentinvention through detailed examinations of the droop characteristicunder various driving conditions of the device.

Next is considered the relationship between the droop rate and thethickness of the selective oxidation layer of vertical cavity surfaceemitting laser devices having the lowest threshold current at 25° C. orless, with reference to FIGS. 25 and 26. As for the selective oxidationlayers having a thickness of 30 nm or 31 nm, their distributions overlapwith each other. However, when the selective oxidation layers having athickness of 30 nm or 31 nm are compared to the selective oxidationlayers having a thickness of 34 nm, it is understood that the thickerthe selective oxidation layers, the larger the droop rate (closer to 0),and therefore, the negative droop characteristic is suppressed. Thedashed line A in FIGS. 25 and 26 represents the average droop rate ofthe devices whose selective oxidation layers have a thickness of 34 nmand which have the lowest threshold current at 25° C. or less. Thedashed line B represents the average droop rate of the devices whoseselective oxidation layers have a thickness of 30 nm or 31 nm and whichhave the lowest threshold current at 25° C. or less. These resultsindicate that, as described above, the thicker the selective oxidationlayer, the larger the optical confinement coefficient of the oxidizedlayer, and therefore, the fundamental transverse mode becomes stableeven during changes in temperature.

As mentioned above, the droop rate at which the negative droopcharacteristic starts to influence the image quality is roughly −10%. Ifthe droop rate becomes smaller than −10%, it is highly likely that apart of an image formed becomes blurred. As shown in FIG. 25, in thecase where the optical output is 1.4 mW, the average droop rate of thedevices whose selective oxidation layers have a thickness of 34 nm isabout −3%, although the droop rate varies more or less. On the otherhand, the average droop rate of the devices whose selective oxidationlayers have a thickness of 30 nm or 31 nm is about −5%. Based on thedifference in these average droop rates, a droop rate of −10% or morecan be achieved if the thickness of the selective oxidation layer is 25nm or more.

Also as shown in FIG. 26, in the case where the optical output is 0.3mW, the average droop rate of the devices whose selective oxidationlayers have a thickness of 34 nm is about −5%, although the droop ratevaries more or less. On the other hand, the average droop rate of thedevices whose selective oxidation layers have a thickness of 30 nm or 31nm is about −7%. Based on the difference in these average droop rates, adroop rate of about −10% or more can be achieved if the thickness of theselective oxidation layer is 25 nm or more.

Thus, devices whose selective oxidation layers have a thickness of 25 nmor more and which has the lowest oscillation threshold current at 25° C.or less are capable of achieving a droop rate of about −10% or more overa wide output range from low to high output.

In vertical cavity surface emitting laser devices which has the lowestoscillation threshold current at a temperature higher than roomtemperature (25° C.), the oscillation efficiency increases when thetemperature of the active layer is increased by current injection, andtherefore, the negative droop characteristic appears, as describedabove. This trend is prominent in the case when vertical cavity surfaceemitting laser devices are driven by the pulse current yielding anoptical output of 0.3 mW, as shown in FIG. 26.

As for both the optical confinement coefficient and the temperature atwhich the lowest threshold current is obtained (the amount of detuning),it is important to set them in such a manner that, in order to suppressthe negative droop characteristic, the efficiency (luminous efficiency)of the vertical cavity surface emitting laser device does not increasemore than the efficiency obtained at room temperature when thetemperature of the active layer increases. In addition, even if theselective oxidation layer has a certain degree of thickness, thenegative droop characteristic is likely to appear if the temperature atwhich the lowest threshold current is obtained is set higher.

If a vertical cavity surface emitting laser device having the lowestthreshold current at 25° C. or more is driven by a current pulseyielding an optical output of 0.3 mW, the negative droop characteristicappears prominently, as shown in FIG. 26. However, if the temperature atwhich the lowest threshold current is obtained is 35° C. or less and thethickness of the selective oxidation layer is 30 nm or more, a drooprate of −10% or more is achieved on average.

As shown in FIG. 25, if the vertical cavity surface emitting laserdevice is driven by a current pulse yielding an optical output of 1.4mW, a droop rate of −10% or more is achieved regardless of the thicknessof the selective oxidation layer (30 nm, 31 nm, or 34 nm) over the rangeof temperature of FIG. 25 at which the lowest threshold current isobtained.

In conclusion, a vertical cavity surface emitting laser device whoseselective oxidation layer is 30 nm or more in thickness and which hasthe lowest threshold current at 35° C. or less is able to achieve adroop rate of −10% or more over a wide output range. By using such avertical cavity surface emitting laser device as a writing light sourceof a printer, a high-definition image free from brightness unevennesscan be obtained. Note that, with reference to FIG. 24, vertical cavitysurface emitting laser devices having the lowest threshold current at35° C. have an amount of detuning of about 4 nm at room temperature.

In the case where a vertical cavity surface emitting laser device isused in a writing light source, having a large single fundamentaltransverse-mode output is of great advantage. In order to increase thesingle fundamental transverse-mode output, it is effective to reduce thestrength of the optical confinement. This is incompatible with thereduction of the negative droop characteristic.

Given this factor, with the aim of increasing the single fundamentaltransverse-mode output while maintaining the suppression of the negativedroop characteristic, the inventors of the present invention conductedan in-depth examination of the relationship between the configuration ofthe resonator structure and the optical confinement strength of thevertical cavity surface emitting laser device. As a result, it was foundeffective to provide an optical confinement reducing region to beexplained below in the lower semiconductor DBR (n-type substrate-sidemultilayer reflector) in order to achieve the above aim.

The effect of the optical confinement reducing region is explained next.

Fundamental transverse-mode optical confinement coefficients at roomtemperature (300 K) were calculated for a conventional vertical cavitysurface emitting laser device having no optical confinement reducingregion and vertical cavity surface emitting laser devices having theoptical confinement reducing region. FIG. 27 shows the calculationresults. Each vertical cavity surface emitting laser device used in thecalculation has a 780 nm-band oscillation wavelength, and basicallyincludes a lower semiconductor DBR (n-type substrate-side multilayerreflector) having 40.5 pairs of an n-AlAs low refractive index layer andan Al_(0.3)Ga_(0.7)As high refractive index layer; an uppersemiconductor DBR (p-type emission-side multilayer reflector) having 24pairs of a p-Al_(0.9)Ga_(0.1)As refractive index layer and ap-Al_(0.3)Ga_(0.7)As refractive index layer; and Al_(0.6)Ga_(0.4)Asspacer layers. The active layer has a threefold quantum well structureincluding Al_(0.12)Ga_(0.88)As and Al_(0.3)Ga_(0.7)As layers, and isdisposed in the center of the spacer layers. The selective oxidationlayer is disposed, within the upper semiconductor DBR, at a positioncorresponding to the third node of the standing wave counted from theactive layer. The oxidized layer is 28 nm in thickness and the oxideconfinement diameter is 4 μm.

The conventional vertical cavity surface emitting laser device includesa cylindrical mesa post configuration having a diameter of 25 μm on thelower semiconductor DBR, as shown in FIG. 28. On the other hand, thevertical cavity surface emitting laser device having an opticalconfinement reducing region includes the optical confinement reducingregion adjacent to the lower semiconductor DBR, as shown in FIG. 29.

FIG. 30 shows the configuration of the lower semiconductor DBR of theconventional vertical cavity surface emitting laser device. Eachrefractive index layer has an optical thickness of λ/4. FIG. 31 showsthe configuration of the lower semiconductor DBR of the vertical cavitysurface emitting laser device having the optical confinement reducingregion. The optical confinement reducing region has 3 pairs of a highrefractive index layer having an optical thickness of 3λ/4 and a lowrefractive index layer having an optical thickness of λ/4. Note that theoptical confinement reducing region having pairs of a high refractiveindex layer having an optical thickness of 3λ/4 and a low refractiveindex layer having an optical thickness of λ/4, is also referred to as“optical confinement reducing region A” below.

The conventional vertical cavity surface emitting laser device and thevertical cavity surface emitting laser device having the opticalconfinement reducing region A have the same number of pairs of the highand low refractive index layers in the lower semiconductor DBR. Eachhigh refractive index layer of the optical confinement reducing region Ahas an optical thickness corresponding to an odd multiple of λ/4, whichsatisfies the phase condition of the multiple reflection. Therefore, iffree carrier absorption and the like in the semiconductor materials arenot taken into account, the lower semiconductor DBR of each verticalcavity surface emitting laser device having the optical confinementreducing region A has a reflectance in the vertical direction (Zdirection) equal to that of the lower semiconductor DBR of theconventional vertical cavity surface emitting laser device.

FIG. 27 presents the fundamental transverse-mode optical confinementcoefficients obtained in the case where the optical confinement reducingregion A includes one, two or three pairs. In FIG. 27, “zero” in thenumber of pairs represents the conventional vertical cavity surfaceemitting laser device.

According to FIG. 27, it is understood that the vertical cavity surfaceemitting laser devices having the optical confinement reducing region Ahave lower fundamental transverse-mode optical confinement coefficientscompared to the conventional vertical cavity surface emitting laserdevice. In addition, as the number of pairs in the optical confinementreducing region A increases, the optical confinement coefficientdecreases.

FIG. 32 shows another optical confinement reducing region having pairsof a high refractive index layer (Al_(0.3)Ga_(0.7)As) having an opticalthickness of λ/4 and a low refractive index layer (AlAs) having anoptical thickness of 3λ/4. In this case, each low refractive index layeris formed thicker than that of the conventional vertical cavity surfaceemitting laser device. Note that the optical confinement reducing regionincluding pairs of a high refractive index layer having an opticalthickness of λ/4 and a low refractive index layer having an opticalthickness of 3λ/4, is also referred to as “optical confinement reducingregion B” below.

FIG. 33 presents fundamental transverse-mode optical confinementcoefficients at room temperature (300 K) of vertical cavity surfaceemitting laser devices having the optical confinement reducing region B,along with the fundamental transverse-mode optical confinementcoefficient of the conventional vertical cavity surface emitting laserdevice.

According to FIG. 33, it is understood that the vertical cavity surfaceemitting laser devices having the optical confinement reducing region Bhave lower fundamental transverse-mode optical confinement coefficientscompared to the conventional vertical cavity surface emitting laserdevice, as in the case of providing the optical confinement reducingregion A. In addition, as the number of pairs in the optical confinementreducing region B increases, the optical confinement coefficientdecreases. If the fundamental transverse-mode optical confinementcoefficient is compared between the optical confinement reducing regionsA and B having the same number of pairs, it can be seen that the opticalconfinement reducing region B has a larger decreasing effect than theoptical confinement reducing region A. In the AlGaAs mixed crystal whichis a material of the semiconductor multilayer reflector, AlAs has thehighest thermal conductivity. Therefore, the thermal diffusion in thetransverse direction is favorably enhanced by forming the layers made ofAlAs thick, which facilitates reducing the temperature increase in theactive layer. Accordingly, the temperature increase in the centralportion of the vertical cavity surface emitting laser device is reduced,and therefore, changes in the effective refractive index differencebecome small, whereby it is possible to also obtain an effect ofsuppressing the negative droop characteristic.

FIG. 34 shows yet another optical confinement reducing region havingpairs of a high refractive index layer (Al_(0.3)Ga_(0.7)As) having anoptical thickness of 3λ/4 and a low refractive index layer (AlAs) havingan optical thickness of 3λ/4. In this case, both low and high refractiveindex layers are formed thicker than those of the conventional verticalcavity surface emitting laser device. Note that the optical confinementreducing region including pairs of a high refractive index layer havingan optical thickness of 3λ/4 and a low refractive index layer having anoptical thickness of 3λ/4, is also referred to as “optical confinementreducing region C” below.

FIG. 35 presents fundamental transverse-mode optical confinementcoefficients at room temperature (300 K) of vertical cavity surfaceemitting laser devices having the optical confinement reducing region C,along with the fundamental transverse-mode optical confinementcoefficient of the conventional vertical cavity surface emitting laserdevice.

According to FIG. 35, it is understood that the vertical cavity surfaceemitting laser devices having the optical confinement reducing region Chave lower fundamental transverse-mode optical confinement coefficientscompared to the conventional vertical cavity surface emitting laserdevices, as in the case of providing the optical confinement reducingregion A. In addition, as the number of pairs in the optical confinementreducing region C increases, the optical confinement coefficientdecreases. If the fundamental transverse-mode optical confinementcoefficient is compared among the optical confinement reducing regionsA, B and C having the same number of pairs, it can be seen that theoptical confinement reducing region C has the largest decreasing effect.

Thus, the inventors of the present invention have found that thefundamental transverse-mode optical confinement coefficient can be moreeffectively reduced with thicker high and low refractive index layers inthe optical confinement reducing region, as well as with a larger numberof pairs of the refractive index layers.

It should be particularly noted that the optical confinement decreasingeffect increases the single fundamental transverse-mode output withoutworsening the negative droop characteristic as described below.Generally speaking, a reduction in the optical confinement coefficienthas an advantage in increasing the single fundamental transverse-modeoutput; however, it is expected that the stability of the transversemode becomes inferior and the negative droop characteristic is likely tooccur.

FIG. 36 shows the results of an experiment conducted for examining therelationship between the signal fundamental transverse-mode output andthe droop rate of vertical cavity surface emitting laser devices havinga 780 nm-band oscillation wavelength. The filled circles of FIG. 36represent the results obtained from conventional vertical cavity surfaceemitting laser devices, and the open circles represent the resultsobtained from vertical cavity surface emitting laser devices having theoptical confinement reducing region B. The area of the current passageregion in each vertical cavity surface emitting laser device is 16 μm².The number “28” of FIG. 36 indicates that the selective oxidation layeris 28 nm in thickness, and the number “30” indicates that the selectiveoxidation layer is 30 nm in thickness.

Both the negative droop characteristic and the single fundamentaltransverse-mode output are related to the optical confinementcoefficient, and bear an inverse relationship to each other as shown inFIG. 36. For example, since the stability of the transverse modeincreases with a larger optical confinement coefficient, the negativedroop characteristic is suppressed. In addition, with a larger opticalconfinement coefficient, the confinement of higher-order transversemodes is improved, which facilitates oscillation, and accordingly, thesingle fundamental transverse-mode output decreases. If the opticalconfinement coefficient is the only factor for determining the drooprate and the single fundamental transverse-mode output, the correlationbetween the droop rate and the single fundamental transverse-mode outputcan be represented by one straight line, regardless of the presence orabsence of the optical confinement reducing region.

However, as shown in FIG. 36, depending on whether the opticalconfinement reducing region is provided, the straight line correlationbetween the droop rate and the single fundamental transverse-mode outputchanges. More specifically, a vertical cavity surface emitting laserdevice having the optical confinement reducing region achieves a highersingle fundamental transverse-mode output than one without the opticalconfinement reducing region even when they have the same droop rate.This indicates that the optical confinement reducing region has aneffect of increasing the single fundamental transverse-mode outputwithout influencing the droop rate.

Thus, the inventors of the present invention have newly found that thenegative droop characteristic is effectively reduced by providing theoptical confinement reducing region, setting the amount of detuning insuch a manner that the lowest threshold current is obtained at 25° C. orless, and forming the selective oxidation layer having a thickness of 25nm or more, whereby it is possible to increase the single fundamentaltransverse-mode output while favorably maintaining the droopcharacteristic.

The vertical cavity surface emitting laser device 100 includes the thirdlower semiconductor DBR 103 ₃ between the optical confinement reducingregion and the resonator structure. The function of the third lowersemiconductor DBR 103 ₃ is explained next.

In general, light absorption by free carriers occurs in a semiconductormaterial. The light absorption increases in proportion to the electricfield intensity of the light and the free carrier concentration. Thelight energy absorbed by free carriers becomes the kinetic energy of thefree carriers, and is eventually converted into lattice vibrationalenergy. This leads to an increase in the oscillation threshold currentand a decrease in the external differential quantum efficiency (slopeefficiency).

In each semiconductor DBR, the refractive index layers are superimposedone on top of the other so that light reflected from the interface ofeach refractive index layer is in the same phase and has an oppositephase with respect to the incident light, whereby the semiconductor DBRproduces a strong reflection (high reflectance). At this point, sincereflection occurs only at the interface, the electric field intensity(amplitude) in each refractive index layer does not attenuate andremains constant. During the laser oscillation, a standing wave isgenerated in the electric field distribution of the semiconductor DBR,and a node or an antinode alternately appear for every λ/4 opticalthickness. In a normal semiconductor DBR that includes low and highrefractive index layers each having an optical thickness of λ/4, eachinterface between a low refractive index layer and a high refractiveindex layer is a position corresponding to a node and an antinode of thestanding wave. In a region adjacent to the resonator structure, thestanding wave of the oscillation light has high intensity, and lightabsorption by free carriers is significant in this region.

The decreasing effect on the absorption loss is calculated for the casein which one or more pairs of low and high refractive index layers, eachhaving an optical thickness of λ/4, are provided between the opticalconfinement reducing region and the resonator structure, as shown inFIG. 37. Note that a set of the paired high and low refractive indexlayers is also referred to as “absorption loss reducing layer” below.

Assume here that the absorption loss of the conventional semiconductorDBR (see FIG. 30) is 100%. The results of the calculations show that theabsorption loss is decreased by 23% in the case of providing theabsorption loss reducing layer including one pair of high and lowrefractive index layers, as shown in FIG. 38, compared to the case ofproviding no absorption loss reducing layer (see FIG. 32). In addition,the absorption loss is decreased by 54% in the case of providing theabsorption loss reducing layer including three pairs compared to thecase of providing no absorption loss reducing layer. In the calculation,the n-type carrier concentration is changed in the range of 3 to 5×10¹⁸cm⁻³ and the number of pairs included in the optical confinementreducing region is changed in the range of 1 to 5; however, littlechange is seen in the above-mentioned percentages.

In the vertical cavity surface emitting laser device 100, lightabsorption by free carriers is reduced by providing the absorption lossreducing layer including one pair. That is, in the optical confinementreducing region, each refractive index layer is great in thickness, andtherefore, light absorption by free carriers increases due to theincreased thickness of the optical confinement reducing region as wellas inclusion of a larger number of antinodes of the standing wavebecause of the increased thickness. In this case, if the absorption lossreducing layer including one pair is provided, it is possible to reducethe electric field intensity in the optical confinement reducing region,thereby reducing the absorption loss.

FIG. 39 shows fundamental transverse-mode optical confinementcoefficients obtained for the presence and absence of the absorptionloss reducing layer of one pair. Three different optical confinementreducing regions—optical confinement reducing regions A, B and C (typeA, B and C)—are provided in which each high refractive index layer ismade of Al_(0.3)Ga_(0.7)As and each low refractive index layer is madeof AlAs. The number of pairs included in each optical confinementreducing region is three.

FIG. 39 also shows reduction percentages of the optical confinementcoefficients of each type. A reduction percentage represents a reduction(%) of the optical confinement coefficient of each optical confinementreducing region when the fundamental transverse-mode optical confinementcoefficient of the conventional semiconductor DBR (see FIG. 30) is 100%.

According to FIG. 39, it can be understood that change in the opticalconfinement coefficient produced by providing the absorption lossreducing layer including one pair is only a little.

A comparative evaluation was conducted by manufacturing a verticalcavity surface emitting laser device VCSEL1 having no opticalconfinement reducing region (see FIG. 30); a vertical cavity surfaceemitting laser device VCSEL2 having the optical confinement reducingregion B but not having the absorption loss reducing layer (see FIG.32); and a vertical cavity surface emitting laser device VCSEL3 havingthe optical confinement reducing region B and the absorption lossreducing layer including one pair (see FIG. 38). Note that in eachvertical cavity surface emitting laser device, the oscillationwavelength is in a 780-nm band and the area of the current passageregion is 16 μm². In addition, all the vertical cavity surface emittinglaser devices have the same number of pairs of high and low refractiveindex layers in their lower semiconductor DBRs.

The results of the comparative evaluation show that the vertical cavitysurface emitting laser devices VCSEL2 and VCSEL3 exhibit clear andsubstantially equal increases in the single fundamental transverse-modeoutput compared to the vertical cavity surface emitting laser deviceVCSEL1, as shown in FIG. 40. The number “30” of FIG. 40 indicates thatthe selective oxidation layer is 30 nm in thickness, and the number “28”of FIG. 40 indicates that the selective oxidation layer is 28 nm inthickness.

According to FIG. 40, at the same droop rate, each of the verticalcavity surface emitting laser devices VCSEL2 and VCSEL3 has a singlefundamental transverse-mode output exceeding that of the vertical cavitysurface emitting laser device VCSEL1 by about 0.3 to 0.5 mW.

FIG. 41 shows the relationship between the single fundamentaltransverse-mode output (calculated values) and the fundamentaltransverse-mode optical confinement coefficient (calculated values).Symbols used in FIG. 41 denote the same in FIG. 40. In addition, thenumbers “30” and “28” of FIG. 41 also denote the same as in FIG. 40.

According to FIG. 41, compared to the vertical cavity surface emittinglaser device VCSEL1, the vertical cavity surface emitting laser deviceVCSEL3 clearly has a reduced optical confinement coefficient, therebyexhibiting an increased signal fundamental transverse-mode output.

In addition, compared to the vertical cavity surface emitting laserdevice VCSEL2, the vertical cavity surface emitting laser device VCSEL3has a lower threshold current and a higher external differential quantum(slope efficiency). This is due to a reduction in the absorption loss.

As has been described above, the vertical cavity surface emitting laserdevice 100 of the present embodiment has the resonator structureincluding the active layer 105, and the lower semiconductor DBR 103 andthe upper semiconductor DBR 107 having the resonator structure betweenthem. The upper semiconductor DBR 107 includes the current confinementstructure where the oxidized layer 108 b surrounds the current passageregion 108 a. The oxidized layer 108 b includes at least an oxide, andis generated by oxidizing a part of a selective oxidation layer whichincludes aluminum and has a thickness of 30 nm. Herewith, the currentconfinement structure is able to confine the injected current and thetransverse modes of oscillation light at the same time.

The lower semiconductor DBR 103 is provided on the substrate 101 side inrelation to the resonator structure, and includes the second lowersemiconductor DBR 103 ₂ which functions as an optical confinementreducing region for reducing the transverse-direction opticalconfinement. Accordingly, it is possible to perform high-poweroperations in a single fundamental transverse-mode oscillation whilesuppressing the negative droop characteristic.

In addition, the PL wavelength of the active layer is set to 772 nm whenthe resonance wavelength of the resonator is 780 nm and the amount ofdetuning Δλ₀ at room temperature is set to −2 nm so that the lowestthreshold current is obtained at 17° C. Herewith, the negative droopcharacteristic is further suppressed.

Since in the second lower semiconductor DBR 103 ₂, the low refractiveindex layers 103 a are each provided in such a manner as to have anoptical thickness of 3λ/4, it is possible to set the interface of eachrefractive index layer at a position corresponding to an antinode or anode. Note that the low refractive index layers 103 a of the secondlower semiconductor DBR 103 ₂ do not necessarily have an opticalthickness of 3λ/4 but should have an optical thickness of (2n+1)λ/4,where n is an integer equal to or greater than 1. Accordingly, theinterface of each refractive index layer can be set at a positioncorresponding to an antinode or a node of the standing wave.

In the case of supplying a square wave current pulse having a pulseperiod of 1 ms and a pulse width of 500 μs, (P1−P2)/P2=−0.06. Thus, thenegative droop characteristic is further suppressed.

In addition, the third lower semiconductor DBR 103 ₃ is provided betweenthe second lower semiconductor DBR 103 ₂ and the resonator structure,and it is therefore possible to reduce the absorption loss.

As for an optical scanning apparatus 1010 of the present embodiment,since the light source 14 of the optical scanning apparatus 1010 has thevertical cavity surface emitting laser device 100, the optical scanningapparatus 1010 is capable of performing optical scanning with highaccuracy.

A laser printer 1000 of the present invention includes the opticalscanning apparatus 1010, and is therefore capable of forming a highquality image.

b. Modifications of First Embodiment

According to the above embodiment, the third lower semiconductor DBR 103₃ includes a pair of the low refractive index layer 103 a and the highrefractive index layer 103 b; however, the present invention is notlimited to this case.

In the case where there is no need to take into account any absorptionloss in the above embodiment, the second lower semiconductor DBR 103 ₂may be positioned next to the resonator structure, as shown in FIG. 42.

According to the above embodiment, the second lower semiconductor DBR103 ₂ includes three pairs of the low and high refractive index layers103 a and 103 b; however, the present invention is not limited to thecase.

In the above embodiment, a fourth lower semiconductor DBR 103 ₄ shown inFIG. 43 may be used in place of the second lower semiconductor DBR 103₂. The fourth lower semiconductor DBR 103 ₄ includes three pairs of thelow and high refractive index layers 103 a and 103 b. As in the opticalconfinement reduction region B, each low refractive index layer 103 ahas an optical thickness of λ/4 and each high refractive index layer 103b has an optical thickness of 3λ/4.

In the above embodiment, a fifth lower semiconductor DBR 103 ₅ shown inFIG. 44 may be used in place of the second lower semiconductor DBR 103₂. The fifth lower semiconductor DBR 103 ₅ includes three pairs of thelow and high refractive index layers 103 a and 103 b. As in the opticalconfinement reduction region C, each low refractive index layer 103 ahas an optical thickness of 3λ/4 and each high refractive index layer103 b has an optical thickness of 3λ/4.

In the above embodiment, the light source may include a vertical cavitysurface emitting laser array 500 shown in FIG. 45 as an example, inplace of the vertical cavity surface emitting laser device 100.

In the vertical cavity surface emitting laser array 500, multiple (inthis case, 32) light-emitting parts are arranged on a single substrate.In FIG. 45, the M direction is the main scanning corresponding directionand the S direction is the sub-scanning corresponding direction. Notethat the number of light-emitting parts is not limited to 32.

The vertical cavity surface emitting laser array 500 includeslight-emitting parts of four rows by eight columns, in which the columnsare equally spaced along the T direction, which is tilted from the Mdirection to the S direction, as shown in FIG. 14. More specifically, ifeach light-emitting part is orthographically projected on a hypotheticalline along the S direction, the center point of the light-emitting partis shifted from the center point of the nearest neighboringlight-emitting part in the M direction by a uniform amount of c on thehypothetical line. Thus, in the S direction, the light-emitting partshave equal intervals of d. That is, the thirty-two light-emitting partsare aligned in a two-dimensional array. Note that in this specification,the term “light-emitting part spacing” refers to a center-to-centerspacing between two neighboring light-emitting parts.

In this case the spacing c is 3 μm, the spacing d is 24 μm, and thelight-emitting part spacing X in the M direction (see FIG. 46) is 30 μm.

Each light-emitting part has the same configuration as that of theabove-described vertical cavity surface emitting laser device 100, asillustrated in FIG. 47, which is a cross-sectional view along A-A lineshown in FIG. 46. The vertical cavity surface emitting laser array 500is manufactured in the same manner as described for the vertical cavitysurface emitting laser device 100.

Thus, since including integrated vertical cavity surface emitting laserdevices 100, the vertical cavity surface emitting laser array 500 isable to achieve the same effect as the vertical cavity surface emittinglaser device 100. Especially in the case of taking the arrayconfiguration, variation in the thickness profile of the oxidized layer108 a and variation in the oxidation spread among the light-emittingparts are significantly small. Therefore, various characteristicsincluding the droop characteristic are uniform, and therefore, the drivecontrol is readily performed. Variation in the operating life among thelight-emitting parts is also small, and long operating life is achieved.

According to the vertical cavity surface emitting laser array 500, ifeach light-emitting part is orthographically projected on a hypotheticalline along the sub-scanning corresponding direction, the center point ofthe light-emitting part is shifted from the center point of the nearestneighboring light-emitting part in the M direction by the uniform amountof c on the hypothetical line. Accordingly, by adjusting the lightingtiming of the light-emitting parts, the configuration of the verticalcavity surface emitting laser array 500 is regarded on the photoreceptordrum 1030 as if the light-emitting parts are aligned at equal intervalsin the sub-scanning direction.

Since the spacing c is 3 μm, if the magnification of the optical systemof the optical scanning apparatus 1010 is set to about 1.8 times,high-density writing with a density of 4800 dpi (dots/inch) can beachieved. In addition, a further increase in density can be achieved byincreasing the number of light-emitting parts in the main scanningcorresponding direction, narrowing the spacing d so as to reduce thespacing c in the array configuration, or reducing the magnification ofthe optical system, whereby higher quality printing is achieved. Notethat the writing interval in the main scanning direction is readilycontrolled by adjusting the lighting timing of the light-emitting parts.

In this case, even if the writing dot density is increased, the laserprinter 1000 performs printing without slowing down the printing speed.Or in the case of not changing the writing dot density, the printingspeed can be increased.

A trench between two adjacent light-emitting parts is preferably 5 μm ormore in order to allow each light-emitting part to be electrically andspatially isolated from each other. This is because if the trench is toonarrow, it is difficult to control etching during the manufacturingprocess. Note that the mesa preferably has a size of 10 μm or more(length of one side). This is because if the mesa is too small, heat iskept inside during operations, which may cause characteristicdegradation.

According to the above embodiment, the mesa shape is square on a crosssection perpendicular to the laser oscillation direction; however, thepresent invention is not limited to this case. The mesa shape may bearbitrary, for example, circular, elliptical or rectangular.

The above embodiment describes the case in which the oscillationwavelength of the light-emitting part is in the 780 nm band; however,the present invention is not limited to this case. The oscillationwavelength of the light-emitting part may be changed according to thecharacteristics of the photoreceptor.

The vertical cavity surface emitting laser device 100 and the verticalcavity surface emitting laser array 500 may be used in apparatuses otherthan image forming apparatuses. In such a case, the oscillationwavelength may be changed to a 650 nm band, 850 nm band, 980 nm band,1.3 μm band, 1.5 μm band or the like according to application needs.

In the above embodiment, a vertical cavity surface emitting laser arrayhaving one-dimensionally arranged light-emitting parts similar to thevertical cavity surface emitting laser devices 100 may be used in placeof the vertical cavity surface emitting laser device.

In the above embodiment, the laser printer 1000 is used as an example ofthe image forming apparatus; however, the present invention is notlimited to this case.

For example, the present invention may be an image forming apparatus forprojecting laser light directly onto a medium (e.g. paper), on which acolor is developed with the laser light.

The present invention may be applied to an image forming apparatus usinga silver salt film as an image carrier. In this case, a latent image isformed on the silver salt film by optical scanning, and then visible bya process equivalent to the development process of regular silver halidephotography. The visualized image is transferred to printing paper by aprocess equivalent to the printing process of the regular silver halidephotography. Such an image forming apparatus can be used as an opticalplate making apparatus or an optical plotting apparatus for plotting CTscanned images and the like.

The present invention may be a color printer 2000 having multiplephotoreceptor drums, as an example as shown in FIG. 48.

The color printer 2000 is a tandem-type multi-color printer for forminga full color image by superimposing four colors (black, cyan, magentaand yellow). The color printer 2000 includes a black photoreceptor drumK1, a black charging device K2, a black developing device K4, a blackcleaning unit K5 and a black transfer device K6; a cyan photoreceptordrum C1, a cyan charging device C2, a cyan developing device C4, a cyancleaning unit C5 and a cyan transfer device C6; a magenta photoreceptordrum M1, a magenta charging device M2, a magenta developing device M4, amagenta cleaning unit M5 and a magenta transfer device M6; a yellowphotoreceptor drum Y1, a yellow charging device Y2, a yellow developingdevice Y4, a yellow cleaning unit Y5 and a yellow transfer device Y6; anoptical scanning apparatus 2010; a transfer belt 2080; a fixing unit2030 and the like.

Each photoreceptor drum rotates in the direction of the arrow shown inFIG. 48, and the corresponding charging device, developing device,transfer device and cleaning unit are sequentially disposed around thephotoreceptor drum along the rotation direction. Each charging deviceuniformly charges the surface of the corresponding photoreceptor. Thesurface of the photoreceptor drum charged by the charging device isirradiated with light emitted from the optical scanning apparatus 2010,and a latent image is formed on the photoreceptor drum. Then, a tonerimage is formed on the surface of each photoreceptor drum by thecorresponding developing device. Each transfer device transfers thecorresponding color toner image onto a recording sheet on the transferbelt 2080, and, eventually, the transferred superimposed toner imagesare fixed on the recording sheet by the fixing unit 2030.

The optical scanning apparatus 2010 has a light source for each color,which includes either one of a vertical cavity surface emitting laserdevice similar to the vertical cavity surface emitting laser device 100and a vertical cavity surface emitting laser array similar to thevertical cavity surface emitting laser array 500. Accordingly, theoptical scanning apparatus 2010 produces an effect similar to that ofthe optical scanning apparatus 1010. The color printer 2000 includes theoptical scanning apparatus 2010, and therefore is able to produce aneffect similar to that of the laser printer 1000.

The color printer 2000 may cause color misregistration due tomanufacturing errors, positional errors and the like. Even in such acase, if each light source of the optical scanning apparatus 2010includes a vertical cavity surface emitting laser array equivalent tothe vertical cavity surface emitting laser array 500, the color printer2000 selects light-emitting parts to be lit, thereby reducing colormisregistration.

c. Second Embodiment

The second embodiment of the present invention aims at providing avertical cavity surface emitting laser device and a vertical cavitysurface emitting laser array that have longer operating life, highluminous efficiency and excellent temperature characteristics.

The vertical cavity surface emitting laser device 100 of the presentembodiment can also be applied to the light source 14 of the laserprinter 1000, as in the first embodiment. In the present embodiment, thesame reference numerals are given to the components which are common tothose of the first embodiment, and their explanations are omitted.

The vertical cavity surface emitting laser device 100 of the presentembodiment is designed to have an oscillation wavelength of 780 nm band,and includes the substrate 101, the lower semiconductor DBR 103, thelower spacer layer 104, the active layer 105, the upper spacer layer106, the upper semiconductor DBR 107 and the contact layer 109.

The substrate 101 is an n-GaAs monocrystalline substrate.

The lower semiconductor DBR 103 includes the first lower semiconductorDBR 103 ₁, the second lower semiconductor DBR 103 ₂ and the third lowersemiconductor DBR 103 ₂, as an example as shown in FIG. 51.

The first lower semiconductor DBR 103 ₁ is laid over a +Z-directionsurface of the substrate 101 with a buffer layer (not shown) interposedbetween them. The first lower semiconductor DBR 103 ₁ includes 36.5pairs of the n-AlAs low refractive index layer 103 a and then-Al_(0.3)Ga_(0.7)As high refractive index layer 103 b. The lowrefractive index layer 103 a has higher thermal conductivity compared tothe high refractive index layer 103 b (see FIG. 65). In order to reduceelectrical resistance, a compositionally graded layer (not shown) isprovided between each two neighboring refractive index layers. In thecompositionally graded layer, the composition is gradually changed fromone to another. It is designed that each refractive index layer has anoptical thickness of λ/4, where λ is an oscillation wavelength, byincluding ½ the thickness of its neighboring compositionally gradedlayer.

The second lower semiconductor DBR 103 ₂ is laid on a +Z-directionsurface of the first lower semiconductor DBR 103 ₁, and includes threepairs of the low refractive index layer 103 a and the high refractiveindex layer 103 b. In order to reduce electrical resistance, acompositionally graded layer (not shown) is provided between each twoneighboring refractive index layers. It is designed that each lowrefractive index layer 103 a has an optical thickness of 3λ/4 byincluding ½ the thickness of its neighboring compositionally gradedlayer, and each high refractive index layer 103 b has an opticalthickness of λ/4 by including ½ the thickness of its neighboringcompositionally graded layer. The second lower semiconductor DBR 103 ₂is a “heat-releasing structure”. The low refractive index layers 103 ain the second lower semiconductor DBR 103 ₂ are “heat-releasing layers”.

The third lower semiconductor DBR 103 ₃ is laid on a +Z-directionsurface of the second lower semiconductor DBR 103 ₂, and includes a pairwhich includes the low refractive index layer 103 a and the highrefractive index layer 103 b. In order to reduce electrical resistance,a compositionally graded layer (not shown) is provided between each twoneighboring refractive index layers. It is designed that each refractiveindex layer has an optical thickness of λ/4 by including ½ the thicknessof its neighboring compositionally graded layer.

Thus, the lower semiconductor DBR 103 includes 40.5 pairs of low andhigh refractive index layers.

The lower spacer layer 104 is laid on a +Z-direction surface of thethird lower semiconductor DBR 103 ₃, and is a non-doped(Al_(0.1)Ga_(0.9))_(0.5)In_(0.5)P layer.

The active layer 105 is laid on a +Z-direction surface of the lowerspacer layer 104, and includes three quantum well layers 105 a and fourbarrier layers 105 b, as an example as shown in FIG. 5. Each quantumwell layer 105 a is made of GaInPAs, which is a composition inducingcompressive strain, and has a band gap wavelength of about 780 nm. Eachbarrier layer 105 b is made of Ga_(0.6)In_(0.4)P, which is a compositioninducing tensile strain.

The upper spacer layer 106 is laid on a +Z-direction surface of theactive layer 105, and is a non-doped (Al_(0.1)Ga_(0.9))_(0.5)In_(0.5)Player.

A section including the lower spacer layer 104, the active layer 105 andthe upper spacer layer 106 is referred to as a resonator structure,which is designed to have an optical thickness of A. The active layer105 is provided in the center of the resonator structure, whichcorresponds to antinodes of the standing wave of the electric field, inorder to achieve a high stimulated emission rate.

Heat generated in the active layer 105 is designed to be released to thesubstrate 101 mainly via the lower semiconductor DBR 103. The back sideof the substrate 101 is attached to a package using a conductiveadhesive or the like, and heat is released from the substrate 101 to thepackage.

The upper semiconductor DBR 107 is laid on a +Z-direction surface of theupper spacer layer 106, and includes 24 pairs of a p-Al_(0.9)Ga_(0.1)Aslow refractive index layer 107 a and a p-Al_(0.3)Ga_(0.7)As highrefractive index layer 107 b. In order to reduce electrical resistance,a compositionally graded layer (not shown) is provided between each twoneighboring refractive index layers. It is designed that each refractiveindex layer has an optical thickness of λ/4 by including ½ the thicknessof its neighboring compositionally graded layer.

In one low refractive index layer of the upper semiconductor DBR 107, ap-AlAs selective oxidation layer having a thickness of 30 nm isinserted. The selective oxidation layer is optically 5λ/4 away from theupper spacer layer 106 and is disposed within the low refractive indexlayer of the third pair counted from the upper spacer layer 106.

The contact layer 109 is a p-GaAs layer laid on a +Z-direction surfaceof the upper semiconductor DBR 107.

Next is a brief description of a method for manufacturing the verticalcavity surface emitting laser device 100 of the present embodiment.

(1) The above-described laminated body is created by a crystal growthmethod, such as metal-organic chemical vapor deposition (MOCVD method)or molecular beam epitaxy (MBE method).

In this step, trimethylaluminium (TMA), trimethyl gallium (TMG) andtrimethyl indium (TMI) are used as the group-III materials, and arsine(AsH₃) gas and phosphine (PH₃) gas are used as the group-V materials. Inaddition, carbon tetrabromide (CBr₄) is used as a p-type dopant, andhydrogen selenide (H₂Se) is used as an n-type dopant.

(2) A square resist pattern, each side of which is 20 μm, is formed onthe surface of the laminated body.

(3) Using the square resist pattern as a photomask, a square columnarmesa is formed by ECR etching using Cl₂ gas. The etching bottom ispositioned in the lower space layer 104. Note that the mesa preferablyhas a size of 10 μm or more (length of one side). This is because if themesa is too small, heat is kept inside during operation, which may causecharacteristic degradation.

(4) The photomask is removed.

(5) The laminated body is heat-treated in water vapor. In this step, Alin the selective oxidation layer is selectively oxidized from theperiphery of the mesa. Accordingly, an unoxidized region 108 a which issurrounded by an AL oxidized layer 108 b is left in the center of themesa. In this manner, an oxidized current confinement structure isformed, in which a pathway of the current for driving a light-emittingpart is limited to the center of the mesa. The unoxidized region 108 afunctions as a current passage region (current injection region).Appropriate conditions of the heat treatment (holding temperature,holding time and the like) are selected based on results of variouspreliminary experiments so that the current passage region 108 a has adesired size.

(6) The protective layer 111 made of SiN or SiO₂ is formed by chemicalvapor deposition (CVD method).

(7) Polyimide 112 is used to planarize the laminated body.

(8) Apertures for p-electrode contact are provided on the top of themesa. In this step, a photoresist mask is provided on the top of themesa, and then, locations on the mesa, at which the apertures are to beformed, are exposed to light to remove the photoresist mask from thelocations. Subsequently, the apertures are formed by buffered HF (BHF)etching the polyimide 112 and the protective layer 111.

(9) A square resist pattern, each side of which is 10 μm, is formed onthe top of the mesa at a region to be a light emitting part, andp-electrode materials are then vapor-deposited. The p-electrodematerials include Cr/AuZn/Au or Ti/Pt/Au and are deposited in multilayerform.

(10) The p-electrode materials over the region to be a light emittingpart are lifted off, whereby a p-electrode 113 is formed.

(11) The back side of the substrate 101 is polished so as to have apredetermined thickness (about 100 μm, for example), and then, ann-electrode 114 is formed. The n-electrode 114 is a multilayer film madeof AuGe/Ni/Au.

(12) The p-electrode 113 and the n-electrode 114 are ohmically connectedby annealing. Accordingly, the mesa becomes a light-emitting part.

(13) The laminated body is cut into chips.

In an experiment, absorption losses were determined for the followingthree examples: Example 1 in which the lower semiconductor DBR 103includes only the first lower semiconductor DBR 103 ₁ with 40.5 pairs ofrefractive index layers (see FIG. 52); Example 2 in which the lowersemiconductor DBR 103 includes the first lower semiconductor DBR 103 ₁with 37.5 pairs of refractive index layers and the second lowersemiconductor DBR 103 ₂ with three pairs of refractive index layers (seeFIG. 53); and Example 3 in which the lower semiconductor DBR 103includes the first lower semiconductor DBR 103 ₁ with 36.5 pairs ofrefractive index layers, the second lower semiconductor DBR 103 ₂ withthree pairs of refractive index layers, and the third lowersemiconductor DBR 103 ₃ with a pair of refractive index layers, as inthe case of the present embodiment. According to the experiment, anincrease in the absorption loss of Example 3 compared to the absorptionloss of Example 1 is about 77%, when an increase in the absorption lossof Example 2 compared to the absorption loss of Example 1 is regarded as100%. That is, it is understood that the third lower semiconductor DBR103 ₃ reduces the increase in the absorption loss by about 23%. Notethat in the case where the third lower semiconductor DBR 103 ₃ includesthree pairs of refractive index layers (see FIG. 54), an increase in theabsorption loss compared to the absorption loss of Example 1 is about46%.

The experiment also shows that, even if the impurity concentration(impurity doping concentration) is changed between 3×10¹⁸ (cm⁻³) and5×10¹⁸ (cm⁻³), the same decreasing effect on the absorption loss isobtained. In addition, although the wavelength is changed, the samedecreasing effect on the absorption loss is obtained. Furthermore, evenif 5 pairs of refractive index layers are provided in the second lowersemiconductor DBR 103 ₂, the same decreasing effect on the absorptionloss is obtained.

The heat resistance of the lower semiconductor DBR 103 of the presentembodiment is 2720 (K/W). On the other hand, the heat resistance ofExample 1 (see FIG. 52) and Example 2 (see FIG. 53) are 3050 (K/W) and2670 (K/W), respectively. Accordingly, it is understood that the thirdlower semiconductor DBR 103 ₃ has little adverse effect on the heatresistance of the lower semiconductor DBR 103.

FIG. 55 shows a relationship between the number of pairs of refractiveindex layers in the third lower semiconductor DBR 103 ₃ and the heatresistance of the lower semiconductor DBR 103. According to FIG. 55, ifthe number of pairs exceeds 5, the heat release effect by the secondlower semiconductor DBR 103 ₂ becomes half or less. Accordingly, thethird lower semiconductor DBR 103 ₃ preferably includes one to fivepairs of refractive index layers.

A heat release layer causes an increase in the absorption loss but alsomay reduce crystallinity of a layer laid on top of the heat releaselayer. If the active layer laid on top (+z direction in this case) ofthe heat release layer has less crystallinity, the luminous efficiencyis decreased. In the case where the lower semiconductor DBR 103 includesonly the second lower semiconductor DBR 103 ₂ with 40.5 pairs ofrefractive index layers, a significant effect of heat release isachieved; however, the crystallinity of the active layer is difficult tomaintain. Therefore, the second lower semiconductor DBR 103 ₂ preferablyincludes one to five pairs of refractive index layers. The third lowersemiconductor DBR 103 ₃ also restores the crystallinity of the activelayer laid on top of the third lower semiconductor DBR 103 ₃, thusreducing an adverse effect on the active layer.

As for a vertical cavity surface emitting laser having an oxidizedcurrent confinement structure, etching is applied during themanufacturing process to obtain a mesa shape or the like in order toprovide electrical or spatial isolation from the surroundings. At thispoint, etching should be performed deeper than the selective oxidationlayer so as to allow selective oxidation of Al. The selective oxidationlayer is in general provided near the active layer of a A-sidesemiconductor DBR (the upper semiconductor DBR disposed on the upperside of the active layer) in order to reduce current spread, and morespecifically, provided at a position corresponding to the first to fifthnode of the laser-light standing wave of the electric field from theactive layer. However, due to problems with controllability of theetching depth, it is difficult to control etching so that the bottom ofetching reaches deeper than the selective oxidation layer but does notreach the lower semiconductor DBR. Especially, to control the etchingdepth across the entire wafer requires not only control of the etchingtime but also uniformity of etching over the wafer surface anduniformity of thickness of the crystal growth layer. Thus, it isextremely difficult from the production standpoint to perform etching insuch a manner that the bottom of etching does not reach the lowersemiconductor DBR.

Given this factor, it has been proposed to make the lower semiconductorDBR two-tier (see Japanese Laid-open Patent Application Publication No.2003-347670, for example). According to the proposal, AlAs having amarkedly higher thermal conductivity than AlGaAs is used for, in thelower semiconductor DBR, most of the low refractive index layersdisposed closer to the substrate, and AlGaAs is used as in theconventional manner for, in the lower semiconductor DBR, low refractiveindex layers disposed closer to the active layer. In this case, however,it is difficult to increase the thermal conductivity of refractive indexlayers disposed near the resonator structure.

According to the vertical cavity surface emitting laser device 100 ofthe present embodiment, the semiconductor DBRs are mainly made of anAlGaAs material and the resonator structure is made of an AlGaInPAsmaterial, which includes In. In this case, the etching rate of theresonator structure can be set lower than the etching rate of thesemiconductor DBRs. Accordingly, it is possible to readily detectwhether the bottom of etching reaches the resonator structure, using anetching monitor. Herewith, it is possible to perform etching up to thevicinity of the center of the resonator structure with high accuracy andreduce the spread of carriers, thus reducing carriers which do notcontribute to oscillation.

As clear from the above explanation, according to the vertical cavitysurface emitting laser device 100 of the present embodiment, the lowersemiconductor DBR 103 is a first semiconductor multilayer reflector andthe upper semiconductor DBR 107 is a second semiconductor multilayerreflector, as described in the appended claims. In addition, the secondlower semiconductor DBR 103 ₂ is a first partial reflector, and thethird lower semiconductor DBR 103 ₃ is a second partial reflector.

The high refractive index layer 103 b is a first layer and the lowrefractive index layer 103 a is a second layer.

As has been described above, the vertical cavity surface emitting laserdevice 100 of the present embodiment has the resonator structureincluding the active layer 105 between the lower semiconductor DBR 103and the upper semiconductor DBR 107, both of which include multiplepairs of low and high refractive index layers. The lower semiconductorDBR 103 includes the first lower semiconductor DBR 103 ₁ including 36.5pairs of refractive index layers; the second lower semiconductor DBR 103₂ including 3 pairs of refractive index layers; and the third lowersemiconductor DBR 103 ₃ including a pair of refractive index layers. Inthe lower semiconductor DBR 103, each pair includes the n-AlAs lowrefractive index layer 103 a having high thermal conductivity and then-Al_(0.3)Ga_(0.7)As high refractive index layer 103 b having thermalconductivity lower than that of the low refractive index layer 103 a.

In the second lower semiconductor DBR 103 ₂, the low refractive indexlayer 103 a is designed to have an optical thickness of 3λ/4 byincluding ½ the thickness of its neighboring compositionally gradedlayer, and the high refractive index layer 103 b is designed to have anoptical thickness of λ/4 by including ½ the thickness of its neighboringcompositionally graded layer.

The second lower semiconductor DBR 103 ₃ is disposed between theresonator structure and the second lower semiconductor DBR 103 ₂. Eachrefractive index layer is designed to have an optical thickness of λ/4by including ½ the thickness of its neighboring compositionally gradedlayer.

According to the above-described configuration, it is possible toincrease the heat release efficiency while reducing an increase in theabsorption loss. Herewith, the vertical cavity surface emitting laserdevice 100 of the present embodiment has longer operating life, highluminous efficiency and excellent temperature characteristics.

As to the optical scanning apparatus 1010 of the present embodiment,since the light source 14 of the optical scanning apparatus 1010 has thevertical cavity surface emitting laser device 100, the optical scanningapparatus 1010 is capable of performing stable optical scanning.

As to the laser printer 1000 of the present embodiment, the laserprinter 1000 is capable of forming a high quality image since includingthe optical scanning apparatus 1010.

In addition, the operating life of the vertical cavity surface emittinglaser device 100 is dramatically increased, which allows the writingunit or the light source unit to be used again.

d. Modifications of Second Embodiment

According to the above embodiment, in the second lower semiconductor DBR103 ₂, the low refractive index layer 103 a is designed to have anoptical thickness of 3λ/4 by including ½ the thickness of itsneighboring compositionally graded layer. However, the present inventionis not limited to this case, and it suffices if the low refractive indexlayer 103 a has an optical thickness satisfying (2n+1)λ/4 (n is aninteger equal to or greater than 1) by including ½ the thickness of itsneighboring compositionally graded layer.

According to the above embodiment, the second lower semiconductor DBR103 ₂ includes three pairs of the low refractive index layer 103 a andthe high refractive index layer 103 b; however, the present invention isnot limited to this case.

According to the above embodiment, the third lower semiconductor DBR 103₃ includes a pair of the low refractive index layer 103 a and the highrefractive index layer 103 b; however, the present invention is notlimited to this case. The third lower semiconductor DBR 103 ₃ mayincludes one to five pairs of the low refractive index layer 103 a andthe high refractive index layer 103 b.

According to the above embodiment, the mesa shape is square on a crosssection perpendicular to the laser oscillation direction; however, thepresent invention is not limited to this case. The mesa shape may be anarbitrary, for example, circular, elliptical or rectangular.

In the above embodiment, the impurity concentration of a part of thelower semiconductor DBR 103 adjacent to the resonator structure may belowered as compared to the remaining part. The amount of absorptionincreases with an increase in the impurity concentration. Therefore, theimpurity concentration of a part subjected to an influence of theabsorption to a greater degree is made lower than that of a part havingless influence of the absorption. In this way, it is possible to reducethe increase in the absorption, which is produced by increasing thethickness of the low refractive index layer. For example, the impurityconcentration of four pairs in the lower semiconductor DBR 103 adjacentto the resonator structure may be 5×10¹⁷ (cm⁻³), and the impurityconcentration of the remaining pairs may be 1×10¹⁸ (cm⁻³).

The above embodiment describes the case in which the oscillationwavelength of the light-emitting part is in the 780 nm band; however,the present invention is not limited to this case. The oscillationwavelength of the light-emitting part may be changed according to thecharacteristics of the photoreceptor.

The vertical cavity surface emitting laser device 100 may be used inapparatuses other than image forming apparatuses. In such a case, theoscillation wavelength may be changed to a 650 nm band, 850 nm band, 980nm band, 1.3 μm band, 1.5 μm band or the like according to applicationneeds.

As an example, FIG. 56 shows a vertical cavity surface emitting laserdevice 100A designed to have an oscillation wavelength of 850 nm band.

The vertical cavity surface emitting laser device 100A includes asubstrate 201, a lower semiconductor DBR 203, a lower spacer layer 204,an active layer 205, an upper spacer layer 206, an upper semiconductorDBR 207, a contact layer 209 and the like.

The substrate 201 is an n-GaAs nomocrystalline substrate.

The lower semiconductor DBR 203 includes a first lower semiconductor DBR203 ₁, a second lower semiconductor DBR 203 ₂ and a third lowersemiconductor DBR 203 ₃, as an example as shown in FIG. 57.

The first lower semiconductor DBR 203 ₁ is laid over a +Z-directionsurface of the substrate 201 with a buffer layer (not shown) interposedbetween them. The first lower semiconductor DBR 203 ₁ includes 30.5pairs of an n-AlAs low refractive index layer 203 a and ann-Al_(0.1)Ga_(0.9)As high refractive index layer 203 b. The lowrefractive index layer 203 a has higher thermal conductivity compared tothe high refractive index layer 203 b (see FIG. 65). In order to reduceelectrical resistance, a compositionally graded layer (not shown) isprovided between each two neighboring refractive index layers. In thecompositionally graded layer, the composition is gradually changed fromone to another. It is designed that each refractive index layer has anoptical thickness of λ/4, where λ is an oscillation wavelength, byincluding ½ the thickness of its neighboring compositionally gradedlayer.

The second lower semiconductor DBR 203 ₂ is laid on a +Z-directionsurface of the first lower semiconductor DBR 203 ₁, and includes fivepairs of the low refractive index layer 203 a and the high refractiveindex layer 203 b. In order to reduce electrical resistance, acompositionally graded layer (not shown) is provided between each twoneighboring refractive index layers. It is designed that each lowrefractive index layer 203 a has an optical thickness of 3λ/4 byincluding ½ the thickness of its neighboring compositionally gradedlayer, and each high refractive index layer 203 b has an opticalthickness of λ/4 by including ½ the thickness of its neighboringcompositionally graded layer.

The third lower semiconductor DBR 203 ₃ is laid on a +Z-directionsurface of the second lower semiconductor DBR 203 ₂, and includes a pairof the low refractive index layer 203 a and the high refractive indexlayer 203 b. In order to reduce electrical resistance, a compositionallygraded layer (not shown) is provided between each two neighboringrefractive index layers. It is designed that each refractive index layerhas an optical thickness of λ/4 by including ½ the thickness of itsneighboring compositionally graded layer.

The lower spacer layer 204 is laid on a +Z-direction surface of thethird lower semiconductor DBR 203 ₃, and is a non-dopedAl_(0.4)Ga_(0.6)As layer.

The active layer 205 is laid on a +Z-direction surface of the lowerspacer layer 204, and includes three quantum well layers 205 a and fourbarrier layers 205 b, as an example as shown in FIG. 12. Each quantumwell layer 205 a is made of Al_(0.12)Ga_(0.88)As, and each barrier layer205 b is made of Al_(0.3)Ga_(0.7)As.

The upper spacer layer 206 is laid on a +Z-direction surface of theactive layer 205, and is a non-doped Al_(0.4)Ga_(0.6)As layer.

A section including the lower spacer layer 204, the active layer 205 andthe upper spacer layer 206 is referred to as a resonator structure,which is designed to have an optical thickness of λ. The active layer205 is provided in the center of the resonator structure, whichcorresponds to an antinode of the standing wave of the electric field,in order to achieve a high stimulated emission rate. Heat generated inthe active layer 105 is designed to be released to the substrate 201mainly via the lower semiconductor DBR 203.

The upper semiconductor DBR 207 is laid on a +Z-direction surface of theupper spacer layer 206, and includes 24 pairs of a p-Al_(0.9)Ga_(0.1)Aslow refractive index layer 207 a and a p-Al_(0.1)Ga_(0.9)As highrefractive index layer 207 b. In order to reduce electrical resistance,a compositionally graded layer (not shown) is provided between each twoneighboring refractive index layers. It is designed that each refractiveindex layer has an optical thickness of λ/4 by including ½ the thicknessof its neighboring compositionally graded layer.

In one low refractive index layer of the upper semiconductor DBR 207, ap-AlAs selective oxidation layer having a thickness of 30 nm isinserted. The selective oxidation layer is disposed at a positionoptically λ/4 away from the upper spacer layer 206.

The contact layer 209 is a p-GaAs layer laid on a +Z-direction surfaceof the upper semiconductor DBR 207.

The vertical cavity surface emitting laser device 100A is manufacturedin the same manner as described for the vertical cavity surface emittinglaser device 100. Note that in FIG. 56, reference numeral 211 denotes aprotective layer; reference numeral 212 denotes polyimide; referencenumeral 213 denotes a p-electrode; reference numeral 214 denotes ann-electrode; reference numeral 208 a denotes an oxidized layer; andreference numeral 208 b denotes a current passage region. The verticalcavity surface emitting laser device 100A is able to achieve the sameeffect as the vertical cavity surface emitting laser device 100.

In this embodiment also, the light source 14 may include the verticalcavity surface emitting laser array 500 shown in FIG. 45 as an example,in place of the vertical cavity surface emitting laser device 100 of thepresent embodiment.

In general, a desired etching depth is different for each light-emittingpart due to variation in the thickness of the crystal growth layer andvariation in the etching rate over the substrate (wafer) surface.However, it is difficult to control etching for all the light-emittingparts so that the bottom of etching reaches deeper than the selectiveoxidation layer but does not reach a low refractive index layer of thelower semiconductor DBR, which low refractive index layer has an Alcomponent equivalent to that of the selective oxidation layer.

Especially in the case of a vertical cavity surface emitting laserarray, if an etching width is different due to a difference in thelight-emitting part spacing, the etching rate changes. In such a case,even if the above variations are not present, the etching depth ischanged for each light-emitting part.

According to the vertical cavity surface emitting laser array 500, thesemiconductor DBR is mainly made of an AlGaAs material and the resonatorstructure is made of an AlGaInPAs material, which includes In.Accordingly, the etching rate of the resonator structure can be setlower than the etching rate of the semiconductor DBRs. Herewith, acrossthe wafer and the array chip, the bottom of etching does not reach thelower semiconductor DBR, and accordingly, it is possible to controletching so that the bottom of etching remains in the resonatorstructure.

Thus, according to the vertical cavity surface emitting laser array 500,etching is controlled by not providing a layer for stopping etching at apredetermined position but slowing the etching rate. Accordingly,etching can be readily controlled since it is possible to readily detectwhether the bottom of etching reaches the resonator structure, using anetching monitor. In addition, it is possible to perform etching up tothe vicinity of the center of the resonator structure with high accuracyand reduce the spread of carriers, thus reducing carriers which do notcontribute to oscillation.

In the case of providing a layer for stopping etching at a predeterminedposition, etching in the depth direction (in this case, −Z direction)can be controlled; however, etching in the lateral direction (in thiscase, a direction parallel to the X-Y plane) cannot be controlled, whichcauses problems, such as lot-to-lot variation in the mesa size.

The vertical cavity surface emitting laser array 500 is a multibeamlight source with 32 channels; however, heat interference of eachlight-emitting part to neighboring light-emitting parts is mitigatedsince heat releasing measures are provided. Accordingly, when multiplelight-emitting parts are driven at the same time, the vertical cavitysurface emitting laser array 500 has only a small characteristic changeand exhibits a longer operating life.

The present embodiment is described using the laser printer 1000 as anexample of the image forming apparatus; however, the present embodimentof the present invention is not limited to this case. As described inthe first embodiment, the image forming apparatus of the presentembodiment may be an image forming apparatus for projecting laser lightdirectly onto a medium (e.g. paper), on which colors are developed withthe laser light; an image forming apparatus using a silver salt film asan image carrier; or the color printer 2000 having multiplephotoreceptor drums.

FIG. 59 shows a schematic structure of an optical transmission system3000. In the optical transmission system 3000, an optical transmittermodule 3001 and an optical receiver module 3005 are connected by anoptical fiber cable 3004, thereby enabling unidirectional opticalcommunication from the optical transmitter module 3001 to the opticalreceiver module 3005.

The optical transmitter module 3001 includes a light source 3002 and adrive circuit 3003 for modulating the light intensity of the laser lightemitted from the light source 3002 according to an electrical signalinput from the outside.

The light source 3002 includes a vertical cavity surface emitting laserarray 600, as an example as shown in FIG. 60.

The vertical cavity surface emitting laser array 600 includes multiple(in this case, ten) light-emitting parts aligned on a single substratein one dimension. Note that the number of light-emitting parts is notlimited to ten.

Each light-emitting part of the vertical cavity surface emitting laserarray 600 is designed to be a vertical cavity surface emitting laserhaving an oscillation wavelength of 1.3 μm band. As illustrated in FIG.61 which is a sectional view along A-A line shown in FIG. 60, eachlight-emitting part includes a substrate 301, a lower semiconductor DBR303, a lower spacer layer 304, an active layer 305, an upper spacerlayer 306, an upper semiconductor DBR 307, a contact layer 309 and thelike.

The substrate 301 is an n-GaAs monocrystalline substrate.

The lower semiconductor DBR 303 includes a first lower semiconductor DBR303 ₁, a second lower semiconductor DBR 303 ₂ and a third lowersemiconductor DBR 303 ₃, as an example as shown in FIG. 62.

The first lower semiconductor DBR 303 ₁ is laid over a +Z-directionsurface of the substrate 301 with a buffer layer (not shown) interposedbetween them. The first lower semiconductor DBR 303 ₁ includes 30.5pairs of an n-AlAs low refractive index layer 303 a and an n-GaAs highrefractive index layer 303 b. The low refractive index layer 303 a hashigher thermal conductivity compared to the high refractive index layer303 b. In order to reduce electrical resistance, a compositionallygraded layer (not shown) is provided between each two neighboringrefractive index layers. In the compositionally graded layer, thecomposition is gradually changed from one to another. It is designedthat each refractive index layer has an optical thickness of λ/4, whereλ is an oscillation wavelength, by including ½ the thickness of itsneighboring compositionally graded layer.

The second lower semiconductor DBR 303 ₂ is laid on a +Z-directionsurface of the first lower semiconductor DBR 303 ₁, and includes fivepairs of the low refractive index layer 303 a and the high refractiveindex layer 303 b. In order to reduce electrical resistance, acompositionally graded layer (not shown) is provided between each twoneighboring refractive index layers. It is designed that each lowrefractive index layer 303 a has an optical thickness of 3λ/4 byincluding ½ the thickness of its neighboring compositionally gradedlayer, and each high refractive index layer 303 b has an opticalthickness of λ/4 by including ½ the thickness of its neighboringcompositionally graded layer.

The third lower semiconductor DBR 303 ₃ is laid on a +Z-directionsurface of the second lower semiconductor DBR 303 ₂, and includes a pairof the low refractive index layer 303 a and the high refractive indexlayer 303 b. In order to reduce electrical resistance, a compositionallygraded layer (not shown) is provided between each two neighboringrefractive index layers. It is designed that each refractive index layerhas an optical thickness of λ/4 by including ½ the thickness of itsneighboring compositionally graded layer.

The lower spacer layer 304 is laid on a +Z-direction surface of thethird lower semiconductor DBR 303 ₃, and is a non-doped GaAs layer.

The active layer 305 is laid on a +Z-direction surface of the lowerspacer layer 304, and includes three quantum well layers 305 a and fourbarrier layers 305 b, as an example as shown in FIG. 63. Each quantumwell layer 305 a is made of GaInNAs, and each barrier layer 305 b ismade of GaAs.

The upper spacer layer 306 is laid on a +Z-direction surface of theactive layer 305, and is a non-doped GaAs layer.

A section including the lower spacer layer 304, the active layer 305 andthe upper spacer layer 306 is referred to as a resonator structure,which is designed to have an optical thickness of λ. The active layer305 is provided in the center of the resonator structure, whichcorresponds to an antinode of the standing wave of the electric field,in order to achieve a high stimulated emission rate. Heat generated inthe active layer 305 is designed to be released mainly via the lowersemiconductor DBR 303.

The upper semiconductor DBR 307 is laid on a +Z-direction surface of theupper spacer layer 306, and includes 26 pairs of a low refractive indexlayer 307 a and a p-GaAs high refractive index layer 307 b. In order toreduce electrical resistance, a compositionally graded layer (not shown)is provided between each two neighboring refractive index layers. It isdesigned that each refractive index layer has an optical thickness ofλ/4 by including ½ the thickness of its neighboring compositionallygraded layer.

In one low refractive index layer of the upper semiconductor DBR 307, ap-AlAs selective oxidation layer having a thickness of 20 nm isinserted. The selective oxidation layer is disposed at a positionoptically 5λ/4 away from the upper spacer layer 306.

The low refractive index layer including the selective oxidation layeris a p-Al_(0.6)Ga_(0.4)As layer, and other low refractive index layersare p-Al_(0.9)Ga_(0.1)As layers. In the low refractive index layerincluding the selective oxidation layer, a p-Al_(0.8)Ga_(0.2)Asintermediate layer (not shown) having a thickness of 35 nm is disposedadjacent to the selective oxidation layer.

The vertical cavity surface emitting laser array 600 is manufactured inthe same manner as described for the vertical cavity surface emittinglaser device 100. Note that in FIG. 61, reference numeral 311 denotes aprotective layer; reference numeral 312 denotes polyimide; referencenumeral 313 denotes a p-electrode; reference numeral 314 denotes ann-electrode; reference numeral 308 a is an oxidized layer; and referencenumeral 308 b is a current passage region.

The vertical cavity surface emitting laser array 600 produces an effectsimilar to that of the vertical cavity surface emitting laser array 500since the lower semiconductor DBR 303 of each light-emitting part has astructure similar to that of the low semiconductor DBR 103 of thevertical cavity surface emitting laser device 100.

Note that when the mesa is formed, a GaInP layer which includes In(indium) may be used in place of the GaAs spacer layer in order to stopetching within the resonator structure.

An optical signal output from the light source 3002 enters and passesthrough the optical fiber cable 3004, and is then input to the opticalreceiver module 3005. The optical fiber cable 3004 includes multipleoptical fibers corresponding one-to-one with the multiple light-emittingparts of the vertical cavity surface emitting laser array 600, as anexample as shown in FIG. 64.

The optical receiver module 3005 includes a light receiving element 3006for converting an optical signal into an electrical signal and areceiving circuit 3007 for performing signal amplification,waveform-shaping and the like on the electrical signal output from thelight receiving element 3006.

The optical transmitter module 3001 of the present embodiment is capableof producing a stable optical signal since the light source 3002includes the vertical cavity surface emitting laser array 600. As aresult, the optical transmission system 3000 is able to performhigh-quality optical transmission.

Accordingly, the optical transmission system 3000 is also effective forshort distance data communication used in home, office, devices and thelike.

In addition, since multiple light-emitting parts having uniformcharacteristics are mounted on a single substrate, data transmissionsimultaneously using a great number of beams can be readily achieved,thereby enabling high-speed communication.

Furthermore, since the vertical cavity surface emitting laser operateswith low power consumption, it is possible to reduce the temperatureincrease especially in the case when the vertical cavity surfaceemitting laser is integrated and used in a device.

Note that the above embodiment describes the case where thelight-emitting parts correspond one-to-one with the optical fibers;however, multiple vertical cavity surface emitting laser devices havingdifferent oscillation wave lengths may be arranged in one dimension orin a two-dimensional array so as to perform multiple-wavelengthtransmission, thereby further increasing the transmission rate.

A unidirectional communication structure is shown above as an example;however, the present invention is also applicable to a bidirectionalcommunication structure.

INDUSTRIAL APPLICABILITY

As has been described above, the vertical cavity surface emitting laserdevice and the vertical cavity surface emitting laser array of thepresent invention are capable of suppressing the negative droopcharacteristic and performing high-power operations in singlefundamental transverse-mode oscillation. The optical scanning apparatusof the present invention is capable of performing optical scanning witha high degree of accuracy. The image forming apparatus of the presentinvention is capable of forming high-quality images.

Also, the vertical cavity surface emitting laser device and the verticalcavity surface emitting laser array of the present invention are capableof achieving longer operating life, high luminous efficiency andexcellent temperature characteristics. The optical scanning apparatus ofthe present invention is capable of performing stable optical scanning.The image forming apparatus of the present invention is capable offorming high-quality images. The optical transmission module of thepresent invention is capable of producing a stable optical signal. Theoptical transmission system of the present invention is capable ofperforming high-quality optical transmission.

This application is based upon and claims the benefit of priority ofJapanese Patent Applications No. 2008-120062 filed on May 2, 2008, No.2008-152427 filed on Jun. 11, 2008 and No. 2009-093021 filed on Apr. 7,2009, the entire contents of which are hereby incorporated herein byreference.

1-12. (canceled)
 13. A vertical cavity surface emitting laser devicethat emits light orthogonally in relation to a substrate, the verticalcavity surface emitting laser device comprising: a resonator structureincluding an active layer; and semiconductor multilayer reflectorsdisposed in such a manner as to sandwich therebetween the resonatorstructure and including a plurality of pairs of a first layer and asecond layer, the first layer and the second layer having differentindexes, wherein the second layer has higher thermal conductivity thanthe first layer, the semiconductor multilayer reflectors include a firstpartial reflector and a second partial reflector, the first partialreflector including at least one of the pairs, in which the second layeris greater in optical thickness than the first layer, the second partialreflector being disposed between the first partial reflector and theresonator structure, and including at least one of the pairs, in whicheach of the first layer and the second layer is less in the opticalthickness than the second layer of the first partial reflector.
 14. Thevertical cavity surface emitting laser device as claimed in claim 13,wherein the optical thickness of each of the first layer and the secondlayer of the second partial reflector is ¼ of an oscillation wavelength.15. The vertical cavity surface emitting laser device as claimed inclaim 13, wherein the second partial reflector includes between one andfive of the pairs.
 16. The vertical cavity surface emitting laser deviceas claimed in claim 13, wherein in the first partial reflector, theoptical thickness of the first layer is ¼ of the oscillation wavelength,and the optical thickness of the second layer is (2n+1)λ/4 where λ isthe oscillation wavelength and n is an integer equal to or greaterthan
 1. 17. The vertical cavity surface emitting laser device as claimedin claim 13, wherein the second layer of each of the pairs is a layerincluding AlAs.
 18. The vertical cavity surface emitting laser device asclaimed in claim 13, wherein the semiconductor multilayer reflectorsinclude a first semiconductor multilayer reflector laid on one side ofthe resonator structure; and a second semiconductor multilayer reflectorlaid on the other side of the resonator structure, heat generated in theactive layer is chiefly released via the first semiconductor multilayerreflector, and the first partial reflector and the second partialreflector are included in the first semiconductor multilayer reflector.19. The vertical cavity surface emitting laser device as claimed inclaim 18, wherein, within the first semiconductor multilayer reflector,impurity doping concentration is relatively lower on a resonatorstructure side.
 20. The vertical cavity surface emitting laser device asclaimed in claim 13, wherein the resonator structure includes indium.21-27. (canceled)